Mission Unaccounted: Japan’s Shift of Role in US Extended Nuclear Deterrence

ABSTRACT Japan’s security policy appears to be undergoing a qualitative change. Policymakers, however, have not provided sufficient explanation to the nation regarding such a shift. What is the qualitative change in Japan’s security policy premised on US extended nuclear deterrence? To answer this question, this article first analyzes changes both in Japanese policymakers’ understanding of US extended nuclear deterrence and the concept of nuclear deterrence, and the shift of Japan’s defense policies. Next, a comparison of deliberations in the National Diet of Japan during the Cold War and after the Cold War, and Japan’s defense policies during the Cold War and after the Cold War, respectively, reveals that while policymakers’ understanding of US extended nuclear deterrence and the concept of nuclear deterrence have not really changed, the security challenges surrounding Japan have shifted in focus from deterrence to response in the recognition of policymakers. With that, Japan’s policy premised on US extended nuclear deterrence has shifted from the stage of mere reliance to the stage of engagement with the United States. This is the qualitative change in Japan’s security policy premised on US extended nuclear deterrence. The implication of the shift is that the use of nuclear weapons itself is a potential policy issue, yet it remains unexamined. The final section points out that Japan’s engagement with the United States in extended nuclear deterrence advances the move toward solidification of US extended nuclear deterrence for Japan. It also discusses Japan’s security challenges associated with the ongoing solidification.


Introduction
During the Cold War in December 1967, Prime Minister Eisaku Sato first announced the Three Non-Nuclear Principles -Japan shall "not possess, not produce, and not permit the introduction" of nuclear weapons -under the pacifism of Article 9 of the Japanese Constitution. They were adopted as the resolution by the National Diet of Japan in November 1971 and have been national policy ever since. On the other hand, Japan's security policy has been premised on US extended nuclear deterrence. The basis for this is the Treaty of Mutual Cooperation and Security between Japan and the United States of America (Japan-US Security Treaty) 1 The relationship between the two states under Japan-US Security Treaty is asymmetric: The United States acts for Japan's security (Article 5), and Japan grants the United States the use of facilities and areas of Japan (Article 6).
Japan became a party to the Treaty on the Non-Proliferation of Nuclear Weapons (NPT) 2 as a non-nuclear-weapon state in June 1976 (Japan 1976). Thus, Article 9 of the Japanese Constitution, the Japan-US Security Treaty, the Three Non-Nuclear Principles, and the NPT formed the framework for Japan's security policy during the Cold War era. Under Japan's security policy, it is the United States that uses nuclear weapons, and Japan has not been required to subjectively consider their use. For Japan, nuclear weapons were a means of influencing the perceptions of its opponent through the possibility of their use rather than their use. It is fair to say that Japan adopted a security policy of relying on US extended nuclear deterrence at the outset with the threat of use of nuclear weapons in mind.
The idea of reliance [tayoru or izon] on US extended nuclear deterrence has been reinforced by the term "nuclear umbrella". As Hans Kristen writes that "'[u]mbrella' sounds so passive" (Kristensen 2006), the concept of the umbrella, which is used to receive and avoid rain, gives an impression to the nation that Japan does nothing just to stand under the umbrella, implying Japan's position of passivity in security policy. This rhetoric of passivity becomes reinforced when it is combined with the narrative that the nuclear umbrella is "hung over" Japan by the United States (Sato 2017, iii, 173). The public understanding of extended nuclear deterrence in Japan seems affected by the rhetoric and narrative, which has partly helped issues surrounding the use of nuclear weapons to not surface in a political debate.
Today, however, Japan's security policy appears to be undergoing a qualitative change. A study by Masakatsu Ota points out that Japan has become an "active stakeholder" in US nuclear policy through the Extended Deterrence Dialogue 3 (Ota 2021, 154). Foreign Minister Toshimitsu Motegi said in a Diet session in April 2020 that there is "no contradiction" between nuclear deterrence as a practical necessity and nuclear disarmament as a future direction. He argued that deterrence and extended deterrence are "necessary, not for right or wrong" but "for the sake of one's own security" in such a severe security environment surrounding Japan (Motegi 2020, 7). Some members of the National Diet have argued that Japan should discuss "nuclear sharing" 4 considering Russia's aggression against Ukraine in February 2022 5 What do these mean in relation to Japan's reliance on US extended nuclear deterrence? Japan and the United States renewed their commitment to strengthening the deterrence and response capabilities of the Japan-US Alliance in the joint statement in May 2022 (Japan and the United States 2022). In this statement, the importance of response is emphasized in the context of strengthening deterrence solidified by 1 Treaty of Mutual Cooperation and Security between Japan and the United States of America (Japan-US Security Treaty), 1960. https://www.mofa.go.jp/region/n-america/us/q&a/ref/1.html. It is noteworthy that there is no reference to nuclear weapons or nuclear deterrence in the treaty. 2 Treaty on the Non-Proliferation of Nuclear Weapons (NPT), 1 July 1968. 729 UNTS 161. https://treaties.un.org/doc/ publication/unts/volume%20729/volume-729-i-10485-english.pdf 3 In the Extended Deterrence Dialogue, Japan and the United States discuss deterrence issues which has been held on a regular basis since 2010 (Japan 2019a). 4 For the concept of NATO's nuclear sharing arrangements, see NATO (2022). 5 For example, former Prime Minister Shinzo Abe suggested Japan start "nuclear sharing" discussion (Asahi Shimbun 2022). Prime Minister Fumio Kishida, however, dismissed such views (Kishida 2022a, 13).
a response capability yet to be increased especially by Japan. This means that Japan, along with the United States which retains response capabilities, will improve its response capabilities. Thus, the threat of use of nuclear weapons by the United States for the security of Japan is expected to become more credible in terms of enhanced nuclear deterrence. What does this mean for Japan's security? Policymakers, however, have not provided sufficient explanation regarding what such an "active stakeholder" position, if any, and the move toward discussing nuclear sharing and strengthening deterrence and response capabilities imply for Japan's security even if they argue that the policy of extended nuclear deterrence is "necessary". As will be seen later, how to respond to the failure of deterrence has become a potential policy issue in the recognition of Japanese policymakers in the face of change of the security environment surrounding Japan. In Northeast Asia, North Korea has acquired nuclear weapons and improved its missile capabilities. China, which has rapidly emerged economically and militarily, is actively expanding into the surrounding sea areas.
Glenn Snyder, who made pioneering contributions to the study of deterrence theory, elaborates in his famous publication Deterrence and Defense the strategic thinking formed during the Cold War. He is first to discuss deterrence strategy in detail and then to consider "defense" in the event of a "failure" of deterrence (Snyder 1961, 3-51). Referring to the difference between deterrence and coercive diplomacy formulated by Alexander George and Richard Smoke, Jack Levy explains "[d]eterrence invokes threats to "dissuade an adversary from initiating an undesired action", while "coercive diplomacy is a response to an action that has already been taken" (Levy 2008, 539). Todd S. Sechser and Matthew Fuhrmann point out in their thorough empirical research that nuclear weapons are useful mainly for "deterrence and self-defense" but not for "coercion" (Sechser and Fuhrmann 2017, iii). In today's security environment surrounding Japan, whether and how Japan that adopts a security policy premised on US extended nuclear deterrence should respond to an action -not inaction -by a threat has surfaced as a security challenge in the recognition of Japanese policymakers.
What, then, is the qualitative change in Japan's security policy premised on US extended nuclear deterrence? To answer this question, this article first analyzes changes both in Japanese policymakers' understanding of US extended nuclear deterrence and the concept of nuclear deterrence, and the shift of Japan's defense policies. The first two sections examine both the record of deliberations in the National Diet of Japan and Japan's defense policies during the Cold War and after the Cold War. Next, a comparison of deliberations in the National Diet during the Cold War and after the Cold War, and the defense policies during the Cold War and after the Cold War, respectively, reveals that while Japanese policymakers' understanding of US extended nuclear deterrence and the concept of nuclear deterrence have not really changed, the security challenges surrounding Japan have shifted in focus from deterrence to response in the recognition of policymakers. With that, Japan's policy premised on US extended nuclear deterrence has shifted from the stage of mere reliance to the stage of engagement with the United States. This is the qualitative change in Japan's security policy premised on extended nuclear deterrence provided by the United States. The implication of the shift is that the use of nuclear weapons itself is a potential policy issue, yet it remains unexamined in deliberations in the National Diet. The final section of this article points out that sharing of roles with the United States advances the move toward solidification of US extended nuclear deterrence for Japan. It also discusses Japan's security challenges associated with the ongoing solidification of extended nuclear deterrence.

Diet Deliberations on the Relationship between Possession of Nuclear Weapons and Article 9 of the Japanese Constitution during the Cold War
In deliberations of the National Diet of Japan during the Cold War, policymakers debated the constitutionality of the possession of nuclear weapons under Article 9 of the Japanese Constitution. On 24 April 1957, Prime Minister Nobusuke Kishi, when asked about Japan-US cooperation over nuclear weapons, replied that the "main focus" is "defense", and that Japan has "not received nor has any intention of receiving offensive weapons" (Kishi 1957, 4). When asked about the constitutionality of possessing nuclear weapons, Akira Kodaki, director general of the Defense Agency, said that "atomic and hydrogen bombs as strategic nuclear weapons" are "contrary to Article 9 of the Constitution" (Kodaki 1957a, 8). Shuzo Hayashi, director-general of the Cabinet Legislation Bureau, said that nuclear weapons "cannot be possessed under the Constitution" at "the present time", but that we "cannot guarantee" what "scientific development" in the "future" will produce (Hayashi 1957, 9).
The following day, on 25 April, the cabinet of Kishi clarified its position by presenting the government's view. Kodaki stated that "what are now called nuclear weapons" appear to be "of an aggressive nature", and "if that is the case, the Constitution does not permit Japan to possess nuclear weapons of this type on its own" (Kodaki 1957c, 1). This left the implication that the constitutionality of non-offensive nuclear weapons, such as "very small warheads as tactical weapons", would be considered "a separate issue" (Kodaki 1957b, 8).
After this debate, the government's position on the constitutionality of possessing nuclear weapons was organized. In March 1964, during the cabinet of Hayato Ikeda, Cabinet Legislation Bureau Director-General Hayashi said that, as an interpretation of Article 9 of the Constitution, it is "not unconstitutional" to possess the "limited forces necessary for self-defense", and that "simply because a nuclear weapon is a nuclear weapon does not immediately mean" that it "exceeds the limited forces necessary for selfdefense under the Constitution" (Hayashi 1964, 20). In December 1965, during the cabinet of Eisaku Sato, Masami Takatsuji, director-general of the Cabinet Legislation Bureau, said that it "cannot be said that Article 9, paragraph 1 permits the possession" of weapons, "whether they are nuclear weapons or not", in the event of an armed attack by another state "unless they are limited to the legitimate purposes and limits of the national defense". He continued that the "possession" of nuclear weapons "would not be considered unconstitutional" if they were "limited to such a limit" (Takatsuji 1965, 23). In March 1978, the cabinet of Takeo Fukuda presented the government's view titled "Interpretation of Article 9 of the Constitution Concerning Possession of Nuclear Weapons", which Hideo Sanada, director-general of the Cabinet Legislation Bureau, explained in the National Diet. To this day, this is the government's position on the possession of nuclear weapons and interpretation of the Japanese Constitution (Sanada 1978, 6).

Diet Deliberation on the Relationship between the Use of Nuclear Weapons and Article 9 of the Constitution during the Cold War
What, then, was the policymaker's understanding of the relationship between the use of nuclear weapons and Article 9 of the Constitution? In April 1957, Prime Minister Kishi said that Japan, with its "actual experiences" of the atomic bombings, cannot defend itself with weapons that "ignore" its national sentiments. Thus, he asserted that he "strongly rejects" the "use [shiyou] of nuclear weapons", atomic and hydrogen bombs (Kishi 1957, 4). In connection with that answer, Defense Agency Director General Kodaki said that, considering the provisions of the Constitution, Japan's national sentiments, and national power, "nuclear weapons may be also used [tsukau]" where the minimum necessary forces for self-defense are "insufficient", but that in such a case, "the basic idea is to rely on the mechanism of collective security" (Kodaki 1957a, 8).
Kishi then pointed out that there are two issues regarding nuclear weapons: the "question of interpretation of the Constitution" and the "political question". He stated that Japan in the cabinet of Kishi "will never arm itself with nuclear weapons", and that the "policy of not allowing nuclear weapons into the country" has been "strictly adhered to" and "will continue to be adhered to" in the future. On the other hand, Kishi expressed the view that "the interpretation of the Constitution that "nuclear weapons cannot be employed [mochiiru] as a violation of the Constitution if they are named nuclear weapons" is "not appropriate". He explained that "no one disputes" that Japan "cannot possess" nuclear weapons "such as the atomic and hydrogen bombs that exist today" under the Constitution. However, he also pointed out that "nuclear weapons today are in the process of development" and that "we do not know how they may develop in the future". He noted this is what "I already stated in the past". This was an answer to a question about the relationship between the possession of nuclear weapons and Article 9 of the Constitution, but he referred to the possibility of "employing" nuclear weapons (Kishi 1959, 2).
During the cabinet of Ikeda in March 1964, when asked whether it was unconstitutional to possess offensive nuclear weapons, Cabinet Legislation Bureau Director-General Hayashi replied that the mere "use" [tsukau] of "nuclear energy" to "kill, wound, or destroy objects in combat" should "not by itself constitute an immediate violation of the Constitution" (Hayashi 1964, 20). This answer, referring to the "use" of "nuclear energy" of nuclear weapons, paves the way for discussion of the constitutionality of the use of nuclear weapons for killing, wounding, and destroying. Since the answer was in response to the question on the constitutionality of the possession of nuclear weapons of an offensive nature, however, it still did not leave the framework of the previous discussions.
When asked what nuclear deterrence is, Sato replied that it has "retaliatory power" [houfukuryoku], with the understanding that such retaliatory power "has something that would force (an opponent) to refrain from a nuclear attack". He continued that since the Japan-US Security Treaty has the "retaliatory power" [houfukuryoku], it "would not be possible to attack Japan so easily" (Sato 1966a, 32). He recognized that an attack on Okinawa or the mainland of Japan could break out "only if an attacking state is prepared" for an overwhelming "nuclear retaliation" by the United States (Sato 1966b, 33). Sato said that "nuclear deterrence" is "something that will not cause war". Because of the powerful "nuclear capabilities of the United States", he said that "there is no such thing as a war against the United States head-on", and that "we trust in this deterrence". (Sato 1968a, 63). Thus, Sato made it clear that Japan "relies" [tayoru] on US nuclear deterrence (Sato 1968b, 13).
Sato also noted that as long as there are nuclear weapons, "we must still be prepared to be attacked by nuclear weapons", and that in the event of a nuclear attack, "the means of protecting" Japan would be the "nuclear retaliatory power" [kaku no houfukuryoku] of the United States (Sato 1968c, 20). However, with regard to US nuclear deterrence, on which he said he "relies", he also acknowledged that "I do not know what kind of nuclear weapons these weapons are" (Sato 1968d, 19). Regarding Japan's relationship with US nuclear strategy, Sato stated that Japan does "not have a hand in" US nuclear strategy. He explained that Japan itself does "not produce nuclear weapons, not possess nuclear weapons, and not permit the introduction of nuclear weapons", although it "relies" [tayorinisuru] on US nuclear deterrence (Sato 1968e, 33).
What, then, were Prime Minister Takeo Miki's thoughts? Miki and US President Gerald Ford for the first time made the Japan-US Joint Announcement to the press in August 1975 regarding the US commitment to providing nuclear deterrence for the security of Japan ("The World and Japan" Database n.d.a). Miki stated earlier in June that "the Japan-US Security Treaty makes Japan's defense capabilities even more secure". Against nuclear threats, he recognized that since "Japan does not develop nuclear weapons", it is "natural" to "rely" [izon] on US nuclear deterrence through the Japan-US Security Treaty (Miki 1975, 683).
The text of the Japan-US Joint Announcement to the press in August 1975 was clarified during the cabinet of Zenko Suzuki. Regarding paragraph 4, Yoshifumi Matsuda, deputy director-general of the Ministry of Foreign Affairs, explained that in the event of an armed attack on Japan, "whether by nuclear or conventional weapons", the president's assurance that the United States will defend Japan "includes measures in all senses of the word", in the sense that "nuclear deterrence or nuclear retaliatory power [kaku no houfukuryoku]" are "not limited to a nuclear attack" against Japan (Matsuda 1982, 22).
Prime Minister Yasuhiro Nakasone raised a discussion about Japan's defense in the event of an invasion while noting that Japan "relies" [izon] on US nuclear deterrence. Nakasone said he has "consistently and clearly stated" that Japan will fulfill its defense while "effectively and appropriately operating" the Japan-US Security Treaty. However, in the event of an invasion against Japan and if the other party were to use nuclear weapons, the government recognizes that "it is necessary to exercise the right of selfdefense to protect the country", and that the Self-Defense Forces (SDF) will "cut off the source of such invasion" both for the sake of the "right of self-defense" and for the "defense of the country". However, he added that "what kind of expression to write for this in the government's answer paper" is a "matter to be studied carefully by the Cabinet Legislation Bureau and related quarters" (Nakasone 1984, 15).
The cabinet of Nakasone also expressed the view that deterrence presupposes the use of nuclear weapons. In December 1985, regarding paragraph 4 of the Japan-US Joint Announcement to the press in August 1975, Hisashi Owada, director-general of the Treaties Bureau in the Ministry of Foreign Affairs, said that it "expresses the essence of deterrence in the (Japan-US) Security Treaty". Owada pointed out that "from the standpoint of deterrence, a commitment to never using nuclear weapons would negate the very effectiveness of deterrence". He said that while nuclear weapons must never be used, "to commit to never using those weapons under any circumstances", or "to completely exclude the possibility of their use", would "undermine effective deterrence". In this sense, Owada recognized that it would "not be appropriate" to "eliminate the possibility of the use of nuclear weapons by the United States" (Owada 1985, 18).  Japan 1972), Japan stated for the first time that it would "rely" [izon] on the "nuclear deterrence of the United States" against nuclear threats (Sato 2017, 50).

Japan's Defense Policy during the Cold War
In October 1976, the cabinet of Masayoshi Ohira decided the first National Defense Program Guidelines for 1977 and beyond (1976 NDPG) (Cabinet of Japan 1976; "The World and Japan" Database n.d.b). Against the backdrop of the détente of the 1970s, the 1976 NDPG was based on an awareness that, in general, there was little likelihood of an all-out military conflict between East and West, and that, in the vicinity of Japan, the balanced relationship between the United States, China, and the Soviet Union, and the existence of the Japan-US Security Treaty, would continue to play an important role in preventing a full-scale invasion against Japan (Japan Ministry of Defense 2019, 208).
Based on the awareness, the 1976 NDPG stated that Japan's defense policy was to possess an "adequate defense capability of its own" while establishing a "posture for the most effective operation of that capability to prevent aggression". It added that a defense posture capable of dealing with any aggression should be constructed, through "maintaining the credibility of the Japan-U.S. security arrangement" and "insuring the smooth functioning of that system". Thus, Japan's defense concept was to "repel limited and small-scale aggression, in principle, without external assistance", while in cases where the unassisted repelling of aggression is not feasible, Japan will continue an unyielding resistance "until such time as cooperation from the United States is introduced". 7 The 1976 NDPG, for the first time, explicitly stated that Japan will "rely" [izon] on the "nuclear deterrent capability of the United States" against "nuclear threat" ("The World and Japan" Database n.d.a).

Japan-US Defense Cooperation during the Cold War
Based on the 1976 NDPG, Japan and the United States reached the first defense cooperation agreement as the Guidelines for the Japan-US Defense Cooperation in November 1978 (1978 Japan-US Guidelines) (Japan and the United States 1978). The 1978 Japan-US Guidelines stated that Japan will "possess defense capability on an appropriate scale" within the scope "necessary for self-defense" and the United States will "maintain a nuclear deterrent capability" and the "forward deployments of combatready forces and other forces" capable of reinforcing them.
When an armed attack against Japan is "imminent", both states will "conduct closer liaison" and will "take necessary measures respectively" and will "make necessary preparations in order to ensure coordinated joint action". While, when an armed attack against Japan "takes place", in principle, Japan by itself will repel "limited, small-scale aggression". When it is difficult to repel aggression, Japan will repel it "with the cooperation of the United States".
Thus, the 1978 Japan-US Guidelines for the first time stated that the United States will "maintain a nuclear deterrent capability" as a posture for preventing aggression. According to Yukio Sato, former senior official of the Ministry of Foreign Affairs, this was "simply a statement of what was natural" under the Japan-US Security Treaty for the United States, which was confronting the Soviet Union's nuclear capabilities. He also commented that the "underlying aim is to prevent Japan from having any interest in nuclear arms" (Sato 2017, 69).

Diet Deliberations on the Relationship between Use of Nuclear Weapons and Article 9 of the Constitution after the Cold War
In deliberations in the National Diet after the Cold War, the focus of debate shifted from the relationship between the possession of nuclear weapons and Article 9 of the Constitution to the relationship between their use and Article 9. The government's position on the relationship between them was first clarified during the cabinet of Ryutaro Hashimoto.
In June 1998, Masasuke Omori, director-general of the Cabinet Legislation Bureau, referring to Sanada's April 1978 statement, said that "constitutional issues on the question of possession of nuclear weapons" would also "fall under the question of their use".
Thus, he stated that it would be "logical" to say that the use of nuclear weapons is "possible" if it is limited to the "minimum necessary" for the defense of Japan (Omori 1998, 13). Later, in March 2016 during the cabinet of Shinzo Abe, Yusuke Yokohata, director-general of the Cabinet Legislation Bureau, confirmed the position of the government by quoting the past statements of Hayashi in 1964, Sanada in 1978, and Omori in 1998(Yokohata 2016. Regarding the use of nuclear weapons under international law 8 , Director-General of the Treaties Bureau Owada stated in July 1984 that nuclear weapons do not conform to the "spirit of humanity" underlying international law. However, he continued that the use of nuclear weapons is "not prohibited" under international law. Owada also mentioned that the "permissible range of the use of nuclear weapons" based on the current provisions of international law will "inevitably emerge" (Owada 1984, 23-24). In its written statement to the ICJ in June 1995, Japan said that the use of nuclear weapons is certainly contrary to the "spirit of humanity" that gives international law its philosophical foundation because of their "immense power to cause destruction, the death of and injury to human beings" (Japan 1995).

Policymakers' Understanding of Japan's Position in US Extended Nuclear Deterrence after the Cold War
The idea, formulated during the Cold War, that Japan relies on US extended nuclear deterrence continued after the Cold War. In September 1991, Toshiki Kaifu, who served as the first prime minister after the Cold War, stated that Japan has employed an "exclusively defense-oriented policy" and was "developing the necessary modest defense capabilities". He also recognized that Japan has "relied" [izon] on the Japan-US Security Treaty for "nuclear deterrence" (Kaifu 1991, 8).
In 1993, the first postwar change of government occurred in Japan. Tsutomu Hata, who served as prime minister in 1994, expressed his view in May 1994 that "US nuclear deterrence" based on the Japan-US Security Treaty is "absolutely necessary" for Japan "to continue to ensure its security" while the international community still embraces destabilizing factors after the end of the Cold War (Hata 1994, 6).
In July 1995, Tomiichi Murayama, who served as prime minister in the coalition government of conservatives and progressives, stated that Japan's "goal has always been the ultimate elimination of nuclear weapons" and that it is important to "take practical steps toward this end, one step at a time". On the other hand, he recognized that "there is no contradiction" between "firmly maintaining" [kenji] the Japan-US Security Treaty to "ensure our own security under its deterrence" and "further nuclear disarmament with the ultimate goal of a world without nuclear weapons" in international society, where a great deal of military power including nuclear weapons exists (Murayama 1995b, 11) In October 2004, Junichiro Koizumi, who served as prime minister in the conservative-centered coalition government, expressed his belief that "deterrence is maintained" [iji] and "will be maintained" by the Japan-US Security Treaty and his "trust" in the United States to "cooperate with Japan in dealing with aggression". Koizumi then continued that Japan must always consider itself to be a "trustworthy partner" of the United States "as an ally" (Koizumi 2004, 38). On the other hand, Koizumi recognized that the security environment surrounding Japan had changed dramatically in the decade or so since the end of the Cold War. Koizumi stated the previous concept of a basic defense force aimed at "deterring" large-scale armed invasions, but that henceforth it was "necessary to shift the point of emphasis" to "responding to new threats and diverse situations" such as terrorism (Koizumi 2005, 2). This marked a shift in the focus of security concerns surrounding Japan.
Naoto Kan, who served as prime minister between 2010-2011 in the Democratic Party of Japan-led coalition government, recognized that "Japan does not have nuclear weapons" and therefore "owes" [ou] its "nuclear deterrence to the US nuclear umbrella" (Kan 2011, 9). Kan also expressed the need to change the way of thinking about defense capabilities, saying that Japan has "come to an era in which it must change" from the previous "static thinking" that "aggression from other states can be deterred" by deploying troops to a "dynamic defense capability" that "shifts the weight" to "certain situations" if they arise (Kan 2010, 15). As will be discussed in detail in the next section, Kan explained the "dynamic defense capability" decided in the NDPG for FY 2011 and beyond (2010 NDPG) (Cabinet of Japan 2010), which replaced the basic defense force of the previous NDPG. The 2010 NDPG aimed to enable more effective deterrence and response to various types of situations and was a "shift in Japan's posture" in relation to deterrence (Sato 2017, 113).
Shinzo Abe, who served as prime minister in the conservative-centered coalition government formed in December 2012 recognized the need to "maintain US nuclear deterrence" under the "Japan-US alliance". He stated that Japan is "relying" [izon] on deterrence, including US nuclear weapons, for its own security in the severe security environment surrounding it (Abe, S 2019, 15). Abe also stated that, from the perspective that the United States is "the only ally that has committed to a treaty obligation" to "act to meet the common danger" in the event of an armed attack against Japan, his government is "closely monitoring" developments in the specific work currently underway regarding the "US nuclear policy review". Abe said that his government is "closely communicating with the United States". (Abe, S 2018, 12). This statement indicates Japan's willingness to be actively involved in the review of US nuclear policy.
Prime Minister Nobuo Kishida recognized that "US extended deterrence, including nuclear deterrence", is "essential", given the security environment surrounding Japan and the fact that nuclear weapons exist. He also stated that Japan and the United States will hold consultations to "maintain and strengthen the credibility" of "US extended deterrence" (Kishida 2022b).

Policymakers' Understanding of Deterrence after the Cold War
What, then, is the deterrence on which the Japanese government says it relies? During the Cold War, Sato expressed his understanding that nuclear deterrence has "retaliatory power" [houfukuryoku] (Sato 1966a(Sato , 32, 1966b. After the Cold War, the cabinet of Murayama expressed the view in March 1995 that "deterrence serves the function of discouraging aggression by making (the other party) clearly aware that it would suffer intolerable damage if it were to commit aggression" (Murayama 1995a, 18-19).
Later, in March 2003, Shigeru Ishiba, director-general of the Defense Agency, explained that "nuclear deterrence and deterrence by conventional weapons" and deterrence by nuclear weapons is distinguished between "deterrence by punishment and deterrence by denial" (Ishiba 2003, 8). The "intolerable damage" indicated in the government's view is conceptually linked to "deterrence by punishment". 9 In April 2005, Secretary of Defense Ohno stated the relationships among deterrence by punishment, deterrence by denial, and Japan's national security policy. He explained that there is "deterrence by punishment", which functions due to the "fear of retaliation if Japan were shot", and "deterrence by denial", which functions due to the calculation that "Japan would be shot with missiles, but they would all be shot down, so it would be a loss". Regarding the two different concepts of deterrence, Ohno stated that Japan's "exclusively defense-oriented" defense policy means that "it cannot be understood as deterrence by punishment", and Japan's view is that "it will be organized as a deterrence by denial" (Ohno 2005, 9). It is unclear however, what the relationship is between "intolerable damage" in the definition of deterrence given by the government in 1995which is conceptually associated with deterrence by punishment -and Ohno's answer that "it cannot be understood as deterrence by punishment".
Also, Ohno, recognizing that the international security environment changed referring to the 9/11 terrorist attacks in 2001, stated that "deterrence is probably a little less effective" than in the past. Regarding the 2004 NDPG, he stated that while "necessary deterrence will of course be secured", he recognized that "effective response to new threats, rather than deterrence", is the key issue (Ohno 2005, 9).

Japan's Defense Policy after the Cold War
This section looks at Japan's defense policies after the Cold War, focusing on Japan's defense concepts, Japan-US security cooperation, and Japan's view on nuclear weapons. The cabinet of Murayama decided the first national defense program guidelines after the Cold War, the NDPG for FY 1996 and beyond (1995 NDPG), in November 1995 (Ministry of Foreign Affairs of Japan 1995). The 1995 NDPG followed the basic defense force presented in the 1976 NDPG 10 , while considering the security environments after the Cold War.
The 1995 NDPG stated: • The security arrangements with the United States are "indispensable" to Japan's security, and the "credibility" of the Japan-US security arrangements is to be "enhanced;" 9 Glenn Snyder defined deterrence as "power to dissuade another party from doing something which one believes to be against one's own interests, achieved by the threat of applying some sanction" (Snyder 1960, 163-164). Alexander George and Richard Smoke also defines deterrence as "the persuasion of one's opponent that the costs and/or risks of a given course of action he might take outweigh its benefits" (George and Smoke 1974, 11). The essence of deterrence in terms of power is an attempt to dissuade an adversary from undertaking an undesired action "not yet initiated" (George and Simons 1994, 7). Deterrence in terms of power can be distinguished between "deterrence by denial" and "deterrence by punishment" (Snyder 1960, 163). In deterrence by denial, an opponent is deterred by showing that the first party has the ability to deny the opponent the benefits of its own actions. In deterrence by punishment, an opponent is deterred by showing that the first party indicates to the opponent the threat of receiving intolerable costs due to its own action. 10 See note 7.
• Against the threat of nuclear weapons, Japan is to "rely" on "US nuclear deterrent" while working "actively" on international efforts for "realistic and steady" nuclear disarmament aiming at a world without nuclear weapons.  Japan 2004) against the background of the need to "deal with new threats and diverse situations", including the proliferation of weapons of mass destruction, ballistic missiles, and international terrorist activities.
The 2004 NDPG stated: • Future defense forces should be capable of "effectively responding" to "new threats and diverse situations" while "maintaining those elements of the Basic Defense Force Concept that remain valid;" • Against the threat of nuclear weapons, Japan will "continue to rely" on the "US nuclear deterrent;" • Japan will play an "active role" in creating a world free of nuclear weapons by taking "realistic step-by-step measures" for nuclear disarmament and non-proliferation; • Japan will also play an "active role" in international disarmament and nonproliferation efforts regarding "other types of weapons of mass destruction and their delivery means, such as missiles." The 2004 NDPG marked a shift from the previous policy of emphasizing the deterrence effect of defense capabilities to that of response capabilities. It continued to "rely" [izon] on US nuclear deterrence against the threat of nuclear weapons. The cabinet of Kan decided the NDPG for FY 2011 and beyond (2010 NDPG) in December 2010 (Cabinet of Japan 2010). It recognized a "global shift in the balance of power" with the "rise of powers such as China, India, and Russia", and "uncertain elements in the Asia-Pacific region", such as disputes over territories and the maritime domain, and issues over the Korean Peninsula and the Taiwan Strait.
The 2010 NDPG stated: • Future defense forces will be built on "Dynamic Defense Force" to "acquire dynamism" to effectively "deter and respond" to various contingences. Japan's defense will "no longer be based" on the "Basic Defense Force Concept," which places priority on ensuring deterrence through the existence of defense forces per se; • To address the threat of nuclear weapons, Japan will play a "constructive and active role" in international nuclear disarmament and non-proliferation efforts toward the "long-term goal" of creating a world without nuclear weapons; • As long as nuclear weapons exist, "extended deterrence provided by the United States, with nuclear deterrent" as a "vital element," will be "indispensable." Japan will "cooperate closely" with the United States to "maintain and improve the credibility" of extended deterrence; • Japan will "appropriately implement its own efforts," including "ballistic missile defense and civil protection." While the 2004 NDPG stated that Japan will "play an active role" in creating a world free of nuclear weapons by taking "realistic step-by-step measures" for nuclear disarmament and non-proliferation, the 2010 NDPG recognized that creating a world without nuclear weapons is the "long-term" goal. In addition, while the 2004 NDPG stated that Japan will "continue to rely" on the "US nuclear deterrent", the 2010 NDPG recognized that US "extended deterrence" with "nuclear deterrent" as a "vital element" that will be "indispensable" as long as nuclear weapons exist. For the first time, the term "extended deterrence" appears, and the terms "rely" or "reliance" in relation to US extended nuclear deterrence are not found in the NDPG. In December 2013, the cabinet of Abe decided the National Security Strategy (NSS) (Cabinet of Japan 2013a), which replaced the Basic National Defense Policy. The NSS stated that Japan has "consistently followed the path of a peace-loving nation" since the end of World War II. It upholds that Japan has adhered to a basic policy of maintaining an "exclusively national defense-oriented policy", "not becoming a military power" that poses a threat to other countries and observing the "Three Non-Nuclear Principles". It also upholds that Japan has consistently engaged in disarmament and non-proliferation efforts, playing a leading role in international initiatives to realize a world free of nuclear weapons.
The NSS defined Japan's national interests as follows: • To maintain its sovereignty and independence; to defend its territorial integrity, to ensure the safety of life, person, and properties of its nationals, and to ensure its survival while maintaining its own peace and security grounded on freedom and democracy; • To achieve the prosperity of Japan and its nationals through economic development, thereby consolidating its peace and security; • To maintain and protect international order based on rules and universal values, such as freedom, democracy, respect for fundamental human rights, and the rule of law 11 It stated: • Japan's defense force is the "final guarantee" of its national security, which "deters direct threats" from reaching Japan and "defeats any threat" that reaches it. Japan will steadily develop its defense force; • With regard to the threat of nuclear weapons, the "extended deterrence" with "US nuclear deterrence" at its core is "indispensable." Japan will "work closely" with the United States to "maintain and enhance the credibility" of the extended deterrence; • Japan will take "appropriate measures" through its own efforts, including ballistic missile defense (BMD) and protection of the people.
The NSS recognized that US extended nuclear deterrence is "indispensable" and Japan will "work closely" with the United States to "maintain and enhance the credibility" of the extended deterrence. The terms "rely" or "reliance" in relation to US extended nuclear deterrence are not found in the document. The cabinet of Abe decided the NDPG for FY 2014 and beyond (2013 NDPG) (Cabinet of Japan 2013b) together with the National Security Strategy. The 2013 NDPG stated that Japan will "efficiently build a highly effective and joint defense force" in light of the increasingly severe security environment surrounding Japan, including North Korea's nuclear and missile development and China's rapidly expanding and intensifying activities in the maritime and aerial domains.
The 2013 NDPG stated: • Japan will build a "Dynamic Joint Defense Force" 12 that can operate seamlessly and flexibly on an ad hoc basis through integrated operations; • Against the threat of nuclear weapons, the "extended deterrence" with US nuclear deterrence at its core is "indispensable." Japan will "closely cooperate" with the United States to "maintain and enhance the credibility" of the extended deterrence; • Japan will take "appropriate responses" through its own efforts, including "ballistic missile defense (NMD) and protection of the people;" • Japan will play a "constructive and active role" in international nuclear disarmament and non-proliferation efforts so as to achieve the "long-term goal" of creating a world free of nuclear weapons.
The 2013 NDPG recognized that US extended nuclear deterrence as a "vital element" will be "indispensable" as long as nuclear weapons exist while the terms "rely" or "reliance" in relation to US extended nuclear deterrence are not found in the document following the 2010 NDPG. The cabinet of Abe decided NDPG for FY 2019 and beyond (2018 NDPG) (Cabinet of Japan 2018). The 2018 NDPG stated that the development of "Multi-Domain Defense Force" will be done while honing the attributes of "Dynamic Joint Defense Force" under the former guidelines.
The 2018 NDPG stated: • "[N]ational defense objectives" are first, "to create . . . security environment desirable for Japan;" second, "to deter threat" from reaching Japan by making opponents realize that doing harm to Japan would be difficult and consequential; and finally, should threat reach Japan, "to squarely counter the threat" and "minimize damage;" • In dealing with the threat of nuclear weapons, "U.S. extended deterrence" with "nuclear deterrence" at its core, is "essential." Japan will "closely cooperate" with the United States to "maintain and enhance its credibility;" • Japan will also increase its own efforts including "comprehensive air and missile defense as well as civil protection" to deal with the threat; • Japan will play an "active and positive role" in nuclear disarmament and nonproliferation towards the "long-term goal" of bringing about a world free of nuclear weapons.
The 2018 NDPG clarified the three objectives of national defense to create a desirable security environment, to "deter" threat and to "counter" the threat. It recognized that "US extended deterrence" with "nuclear deterrence" at its core is "essential" while the terms "rely" or "reliance" in relation to US extended nuclear deterrence are not found in the document after the NPDG 2010.

Japan-US Defense Cooperation after the Cold War
In light of changes in the security environments after the Cold War, Japan and the United States redefined Japan-US security cooperation. As a result, the two states agreed on the Japan-US Joint Declaration on Security Cooperation in April 1996 (Ministry of Foreign Affairs of Japan 1996) and the Guidelines for Japan-US Defense Cooperation (1997 Japan-US Guidelines) (Japan Ministry of Defense 1997). The 1997 Japan-US Guidelines stated: • Both governments will "firmly maintain" existing Japan-US security arrangements. Japan will possess defense capability "within the scope necessary for self-defense" while the United States will "maintain its nuclear deterrent capability, its forward deployed forces" in the Asia-Pacific region, and other forces capable of reinforcing those forward deployed forces; • When an armed attack against Japan is "imminent," the two governments will take steps to "prevent" further deterioration of the situation and make preparations necessary for the defense of Japan; • When an armed attack against Japan "takes place," the two governments will conduct appropriate bilateral actions to "repel" it at the earliest possible stage; • Situations in areas surrounding Japan will have an "important influence" on Japan's peace and security. The concept, "situations in areas surrounding Japan," is "not geographic but situational." The two governments will make every effort, including diplomatic efforts, to prevent such situations from occurring.

Japan and the United States agreed in the Guidelines for Japan-US Defense
Cooperation in April 2015 (2015 Japan-US Guidelines) that the two states will "continuously enhance the Japan-US Alliance" (Japan Ministry of Defense 2015). The 2015 Japan-US Guidelines stated that the United States will "continue to extend deterrence to Japan" through the "full range of capabilities, including US nuclear forces". The 2015 Japan-US Guidelines stated: A. Cooperative Measures from Peacetime Japan and the United States will promote cooperation across a wide range of areas, including through diplomatic efforts, to strengthen the "deterrence and capabilities of the Japan-US Alliance" to ensure the maintenance of Japan's peace and security.

B. Responses to Emerging Threats to Japan's Peace and Security
The Alliance will respond to situations that will have an "important influence on Japan's peace and security." Such situations "cannot be defined geographically. C. Actions in Response to an Armed Attack against Japan "Bilateral actions in response to an armed attack against Japan" remain a "core aspect" of Japan-US security and defense cooperation. When an armed attack against Japan occurs, the two governments will "conduct appropriate bilateral actions" to "repel" it at the earliest possible stage and to "deter any further attacks." D. Actions in Response to an Armed Attack against a Country other than Japan In response to an armed attack against the United States or a third country, the United States and Japan will "cooperate closely" to "respond" to the armed attack and to "deter" further attacks.

Japan's Reliance on United States in Extended Nuclear Deterrence during the Cold War
As are clear from the above examinations of the record of deliberations in the National Diet, policymakers during the Cold War discussed that Japan relies on US extended nuclear deterrence, with the understanding that "nuclear deterrence" is to have the "nuclear retaliation power" (Sato 1968c, 20). What "nuclear retaliatory power" means, however, was unclear. Therefore, during deliberations in the National Diet, policymakers discussed reliance on nuclear retaliatory power in abstract terms (Matsuda 1982, 22), with the threat of use of nuclear weapons by the United States in mind. Why, then, did deliberations in the National Diet remain so abstract? Considering the findings of security studies on the Cold War era and studies on the formation of Japan's postwar security policy, there are three major reasons. First, during the Cold War, the primary security challenge was to deter the Soviet Union, which was perceived as a threat, from taking military action against the United States and its allies. The main strategic focus of extended nuclear deterrence was to deter Soviet armed aggression against NATO, European states in particular, with US nuclear weapons. In comparison, Soviet military threat to Japan was not as grave. Hirofumi Tosaki points out that the United States had "superiority in both conventional and nuclear forces" in East Asia against the Soviet Union, and that the naval and air forces of the United States and Japan were seen as having "high denial capabilities" against an invasion from the Soviet Union across the sea (Tosaki 2009, 56). The perception that the relatively inferior Soviet Union would be deterred by the threat of use of US nuclear weapons allowed Japanese policymakers to consider the meaning of extended nuclear deterrence only in an abstract manner. This situation surrounding Japan did not provide the opportunity for Japanese policymakers to examine the implications for Japan of the extended nuclear deterrence provided by the United States.
China was another concern. Eisaku Sato was the first prime minister to request the US president to provide Japan with nuclear deterrence because of the concern of a nuclear China (USDSOH 1965). China succeeded in its first nuclear test in October 1964 and its first thermonuclear test in June 1967, thus China became the fifth state which has nuclear weapons. However, the government of China "solemnly" declared on the very day of its nuclear test in 1964, "China will never at any time and under any circumstances be the first to use nuclear weapons" (China 1964). This reflected the Chinese strategic thinking formed from the century of humiliation between 1839-1949 that lagging behind means to be exposed to invasion was applied to nuclear weapons as a point of consensus among Chinese leaders led by Mao Zeodong (Xu 2016, 22). As Prime Minister Kakuei Tanaka said "China's nuclear weapons are not a threat" with referring to its declared nuclear policy (Tanaka 1973), the perception that the relatively defensive posture of China in 1970ʹs also allowed Japanese policymakers not to examine the implications of US extended nuclear deterrence for Japan's security.
Second is the background that Japan, having secured its security within the structure given to it by the United States after World War II, has not necessarily had to examine, or has dared to choose to avoid examining, the implications of its reliance on US nuclear weapons in a concrete manner. As Yoshihisa Hara points out, the "solid fait accompli" of Japan-US relations of "domination and subordination" established during the US occupation of Japan following its defeat in World War II has remained the "most basic framework" of relations between the two states to this day (Hara 1991, 33). Kazuya Sakamoto writes that, in the context of the Cold War, cooperation with the United States was the "foundation" of Japan's postwar diplomacy. Under the "choice" of Prime Minister Shigeru Yoshida, who believed that becoming a member of the liberal camp was the "way that best protects Japan's national interests", Sakamoto also writes that Japanese foreign and national security policies have been based on such a "comprehensive subordination system" and "the most fundamental framework" of Japan-US relations (Sakamoto 2020, 28-29). Tetsuya Sakai calls the system "Article 9 = Japan-US Security Treaty Regime [kyujo = ampo taisei]" as the "framework of reference" within which the Japanese government makes the choice of security policy (Sakai 1991, 32). Yukiko Toyoda, thus, argues that Japan has remained in a state of "thought suspension" without considering what deterrence the US military should be to ensure its own national security (Toyoda 2015, 260). Yoshihide Soeya argues that Japanese politics and society are still in a state of "sterile debate and confrontation" over the issue of security, which does not lead to a true strategic debate (Soeya 2016, 13).
Third, the experience of war and the atomic bombings made it difficult to think about security and nuclear weapons issues in Japan. Shingo Yoshida writes that, in postwar Japan, there exists an "anti-militarism" rooted in the "collective memory of the nation" that they became a victim of the disastrous war with the United States due to the military's dictates in the 1930s. Yoshida continues that it has made Article 9 of the Constitution its "fundamental legal foundation" (Yoshida 2012, 9). Political leaders perceived the nation's anti-nuclear sentiment stemming from the experiences of Hiroshima, Nagasaki, and Bikini Atoll as a "nuclear allergy" among the nation (Sato 1967, 6;Fukuda 1972, 9). Koji Murata argues that such "anti-nuclear and anti-war sentiment in Japan" has had a "denial effect" on the governments of Japan and the United States, "not allowing them to do anything", but has not played a "positive role beyond that" enough (Murata 2000, 70). Akira Kurosaki points out that Eisaku Sato "combined" Japan's security policy of relying on US nuclear extended deterrence with the Three Non-Nuclear Principles into the Four Nuclear Policies in January 1968. Sato thus "upgraded" the policy -which had been tacitly understood -to a "declared" one (Kurosaki 2006, 207-209). However, Kurosaki points out Japan's conservative government did "not spontaneously explain" the policy to the public for fear of an outcry (Kurosaki 2006, 188).
The above backgrounds have deprived Japan of a subjective mindset to examine the fundamental question of what nuclear deterrence and extended nuclear deterrence are for Japan. This mindset, which Kurosaki calls "passive reliance on nuclear deterrence" (Kurosaki 2017, 115-116), is inextricably linked to Japan's reliance on the US nuclear umbrella. Heigo Sato writes that Japan has been "reinforcing its nuclear deterrence" with the "Three Non-Nuclear Principles" against the backdrop of a lingering "nuclear allergy" in the nation (Sato 2004, 125), which may point out one aspect of Japan's passive reliance on nuclear deterrence. These backgrounds have led to a lack of awareness of the parties involved in considering Japan's security policy. The lack of awareness underpins the public thinking that the security policy relying on US extended nuclear deterrence is "realistic" and allows its continuation. The reality today is that the "realistic" policy of relying on the US nuclear umbrella has the support of more than 60% of the nation (NHK 2020).

Japan's View on Deterrence after the Cold War
The Japanese government in 1995 expressed the view that deterrence "serves the function of discouraging aggression by making (the other party) clearly aware of the intolerable damage it will suffer if it commits aggression" (Murayama 1995a, 18-19). However, it is still unclear what is exactly the "intolerable damage" of deterrence -which is conceptually linked to "deterrence by punishment".
Nuclear deterrence assumes the use of nuclear weapons (Owada 1985, 18). In the policy of nuclear deterrence, the question is usually posed in the form of what the objective of deterrence is, and the answer is to deter the state. There is no doubt that the state is the object of deterrence. However, "the state" is an abstract concept that refers to a political organization governing a population within a territory, which cannot be the target of physical destruction itself with the use of nuclear weapons. Therefore, even if the state is to be deterred, it is necessary to ask what is to be damaged or destroyed with the use of nuclear weapons. In other words, the question is, what the target is. In terms of what nuclear deterrence is intended to target and inflict "intolerable damage", debates over nuclear deterrence have remained abstract in deliberations in the National Diet even after the Cold War. On the other hand, Japan today is facing shifts in its security agenda.

A Shift in Japan's Security Agenda
The first is the shift concerning security challenges. As mentioned earlier, during the Cold War, the biggest security challenge was to deter the Soviet Union from taking military action against the United States and its allies. After the Cold War, the security environment surrounding Japan has changed. In Northeast Asia, North Korea has acquired nuclear weapons and improved its missile capabilities. China, which has rapidly emerged economically and militarily, is actively expanding into the surrounding sea areas. As examined in the previous section, how to respond to such diverse situations has surfaced as a security challenge in the recognition of Japanese policymakers.
The second is the change of technologies relating to security. Advances in science and technology have brought about significant changes in military strategy. Traditionally, the three domains of land, sea, and air have been the main areas of military activities. The expansion of activities in cyberspace, electromagnetic space, and outer space that accompanies advances in science and technology has directly led to the expansion of the domain of military activities, and so-called multi-domain military strategies are now being considered. Furthermore, the speed and depth of innovation in emerging technologies are having a significant impact on national security. Since emerging technologies can also be applied to the military, for example, the US Department of Commerce identified fourteen categories of emerging technologies that are "critical to US national security" in 2018 (US Department of Commerce 2018). Technological progress in the 21st century is characterized by the remarkable speed and unpredictability of its consequences (Kawai 2020).
The innovation of military technologies can impact nuclear deterrence. Technologies in precision guidance and remote sensing in one state can affect the value of "hardening" and "concealment" technologies of an adversary state, increasing the vulnerability of the adversary's nuclear capabilities (Lieber and Press 2017, 9-13). The resulting instability may affect the motivation for one state to launch a nuclear attack on the other. It may also serve as the motivation for a nuclear arms race between the two. In addition, artificial intelligence can be a factor in strategic instability because of the uncertainty of how it will affect nuclear strategies.

Japan's Shift to Engaging with the United States in Extended Nuclear Deterrence after the Cold War
The above-mentioned changes have prompted the shift in the point of emphasis in Japan's defense policies, which relate to the reasons Japan has been shifting its security posture. The cabinet of Koizumi recognized the need to shift the focus of Japan's security challenges from deterrence to response. The cabinet of Kan indicated response as an element of Dynamic Defense Force alongside deterrence. The National Security Strategy, NDPG, and Japan-US Guidelines all point in the direction of the integration of SDF operations with those of the US military by referring to the importance of "continuously enhancing the Japan-U.S. Alliance". This direction has been ensured by the domestic legislation leading up to the Peace and Security Legislation Development Act in September 2015 13 .
As the focus of national security concerns shifts in the recognition of Japanese policymakers from deterrence to response together with the integration of SDF operations with those of the US military, extended nuclear deterrence, on which policymakers say Japan relies, inevitably entails the issue of response. The question now becomes whether nuclear weapons also have a role in response to diverse situations, and if it does, what role is envisaged for US nuclear capabilities.
The 2015 Japan-US Guidelines stated that the Alliance will "respond" to situations that will have an "important influence on Japan's peace and security". They also stated "the two governments" will "conduct appropriate bilateral actions" to "repel" it at the earliest possible stage and to "deter any further attacks" when an armed attack against Japan occurs. Moreover, they say that the United States and Japan will "cooperate closely" to "respond" to the armed attack and to "deter" further attacks in response to an armed attack against the United States or a third country. The 2018 NPDG stated that Japan will "squarely counter" threats and "minimize damage" should threats reach Japan.
It is unclear from these statements, however, whether nuclear weapons would have a role in response at all. If nuclear weapons would have a role, and if the role would entail their use, a question arises as to how the damage could be minimized when they were used in view of their devastating power and the risk of escalation. It is also a valid question whether it would be even possible to minimize the damage in the first place. These questions related to a role of nuclear weapons in response have not been discussed in deliberations in the National Diet. Japanese policymakers have been discussing US extended nuclear deterrence in abstract terms, with the understanding that "reliance" [tayoru or izon] on US extended nuclear deterrence means inflicting "intolerable damage" on the other party through "nuclear retaliatory power" [kaku no hofukuryoku], and that it is "essential" (Kishida 2022b).
The efforts to "maintain and enhance the credibility" of extended deterrence of the United States with nuclear deterrence (Cabinet of Japan 2013a) have sustained the cooperation between Japan and the United States, which has also led Japan to engaging with the United States in extended nuclear deterrence against the backdrop that the focus of security shifts from deterrence to response. Japan's engagement is manifest in the 2015 Japan-US Guidelines, which is ensured by the domestic legislation of the Peace and Security Legislation Development Act in 2015. The sharing of roles between the two states, whose security policies employ nuclear deterrence respectively, also implies the sharing of roles between both states in extended nuclear deterrence. For Japan, US extended nuclear deterrence has shifted from the stage of mere reliance to the stage of engagement with the United States. This explains the disappearance of the terms "rely" or "reliance" on US extended nuclear deterrence in Japan's NDPG after 2010. This is the qualitative change in Japan's security policy premised on extended nuclear deterrence provided by the United States. However, there have been no explanations from Japanese policymakers of why the shift in the focus of national security concerns is directly linked to the proactive engagement of the US extended nuclear deterrence.

Unexamined Policy Issues of Use of Nuclear Weapons
What are the implications of the shift from the stage of reliance to the stage of engagement? First, it means that Japan now has a motivation to act proactively to enhance the role of nuclear weapons in its security policy. This motivation manifests itself in the Japanese government's opposition to the policy of no first use considered by the United States at the Extended Deterrence Dialogue and on other occasions (Fetter and Wolfsthal 2018, 103;Abe, N 2018, 143-146). Also, such motives can be read from Shinzo Abe's statement that his government will be "closely monitoring" developments in the specific work currently underway regarding the "US nuclear policy review" and "closely communicating with the United States", which is "the only ally that has committed to a treaty obligation" (Abe, N 2018, 12).
As mentioned earlier, Ota points out that Japan has become an "active stakeholder" in US nuclear policy through the Extended Deterrence Dialogue, calling this the move toward "Japan-US nuclear Alliance [nichibei kakudoumeika]" (Ota 2021, 154). Japan's proactive involvement in US extended nuclear deterrence has been quietly moving toward what might be called a "re-redefinition" of the Japan-US Alliance, working in the direction of solidifying Japan's security policy premised on US extended nuclear deterrence.
The position against no first use presupposes the recognition that nuclear weapons are weapons for use. The second implication, then, is that nuclear weapons may be given the status of weapons for use as a means of security policy. As mentioned earlier, in June 1998, Director-General of the Cabinet Legislation Bureau Omori stated that it would be "logical" to say that the use of nuclear weapons is "possible" if it is limited to the "minimum necessary" for the defense of Japan (Omori 1998, 13). Later in March 2016, Director-General of the Cabinet Legislation Bureau Yokohata confirmed this position (Yokohata 2016, 3).
In the context of recent political claims that nuclear sharing should be discussed in Japan, it should be recalled that the interpretation of the Constitution permits Japan to "possess" and "use" nuclear weapons as long as they are within the restrictions of Article 9. Given the interpretation of Article 9 as it has been presented, it may be possible to argue that choosing nuclear sharing as a policy would be permissible, at least under Article 9 of the Constitution, aside from the Three Non-Nuclear Principles and other domestic and international policy restrictions and regulations. Although Japan, as the state that suffered atomic bombings in Hiroshima and Nagasaki, is expected to assume moral responsibility for the abolition of nuclear weapons, it is important to keep in mind that, according to the current interpretation, Article 9 by itself may not be an impediment to the argument that Japan should make the policy choice of nuclear sharing.
Third, the shift from the stage of reliance to the stage of engagement with the United States for Japan in US extended nuclear deterrence implies that the examination of the use of nuclear weapons itself is a potential policy issue. The problem, however, is that the issue of the use of nuclear weapons has not become manifest in policy discussions. The above-mentioned backgrounds have deprived Japan of a subjective mindset regarding the fundamental question of what nuclear deterrence and extended nuclear deterrence are for. Furthermore, the idea of reliance on US nuclear weapons has been reinforced by the term "nuclear umbrella", which has been used to describe Japan's position of passivity. In Japan, issues surrounding the use of nuclear weapons have been concealed by this narrative, and this "threat-biased" understanding of extended nuclear deterrence, which has its origins in the Cold War era, continues to prevent the issue of the use of nuclear weapons from surfacing in policy discussions.

Security Challenges for Japan Associated with Solidification of US Extended Nuclear Deterrence
What challenges does this qualitative change in Japan's security policy premised on US extended deterrence pose for Japan's security? First, it brings a greater risk that Japan becomes a target for the use of nuclear weapons. Russia is a nuclear-weapon state under the NPT with the largest number of nuclear weapons now. In addition, North Korea has acquired nuclear weapons and improved its missile capabilities while China as a nuclearweapon state under the NPT has rapidly emerged economically and militarily and is actively expanding into the surrounding sea areas. In the first place, the US bases in Japan under the Japan-US Security Treaty would become legitimate military targets during hostilities in the event of armed conflict. In addition, while US extended nuclear deterrence is being solidified for Japan against the backdrop of a changing security environment surrounding it, the role of nuclear weapons in response to diverse situations can be focused.
Hirofumi Tosaki points out that it is Japan, not the United States or South Korea, that could be the "primary target" of the use or threat of use of nuclear weapons by North Korea (Tosaki 2016, 145-147). Although North Korea's primary deterrent target is the United States, it is difficult to believe that its own nuclear weapons would be sufficient to deter the United States given North Korea's capabilities. What emerges from this situation, Tosaki analyzes, is that North Korea which does not want Japan to be involved in the event of emergency on the Korean Peninsula could target Japan to coerce Japan not to exercise the right of collective self-defense since Japan could play a complementary role to the United States.
While nuclear deterrence focuses on the prevention of hostile military actions (inaction), it does not provide an answer to the question of how to respond to actions if they cannot be prevented as Hedley Bull points out (Bull 1981, 14). How would Japan respond when deterrence failed and a hostile military action by an adversary would occur? Would there be a role for nuclear weapons of the United States in such a situation? If there would be, what role would they be envisaged to play? Would nuclear weapons be used in such a scenario? If that was the case, it could work toward making Japan a target for the use of nuclear weapons by an adversary. Moreover, changes in military technologies, especially innovations in precision strike and remote detection technologies, could affect the foundations of nuclear deterrence. Such instability could influence the motivation for nuclear attack from one side to the other. This could also work in the direction of increasing the risk of the use of nuclear weapons. Jitsuo Tsuchiyama points out that "if the policy goes wrong", as was the case with the United States after 9/11, "being a strong hegemony creates insecurity, not security". He termed it "security paradox" -the more one works to strengthen security, the more insecure one becomes -(Tuchiyama 2014, 427). Tsuchiyama also points to the history of the "insecurity of a state that possesses the offensive capability of nuclear weapons", which resulted in "prudence" and MAD (mutually assured destruction) as a restraint of mutual power (Tuchiyama 2014, 437). The ongoing solidification of extended nuclear deterrence for Japan runs the risk of the "security paradox". The expansion of military power of the state in the hope of strengthening its security does not necessarily guarantee the increased security. Solidification of extended nuclear deterrence may not bring freedom from fear but in fact can amplify and deepen it as the logic of nuclear deterrence itself has proved (Kawai 2020). The lesson learned from the history is that prudence in mutual restraint comes at a time of heightened security concerns, and an effort of nuclear arms control and disarmament for security reasons, which demonstrates the prudence, is a viable option for ameliorating the situation.
Second, there is the risk of nuclear arms race. The "security dilemma" is that an attempt by one state to strengthen its security is perceived by the other state as an increased threat, resulting in a cycle of insecurity for both states (Herz 1950, 231;Jervis 1978, 167). How the other side perceives the ongoing consolidation of extended nuclear deterrence is a matter of the other side's perception. Japan cannot control the other party's perception. The ongoing consolidation of US extended nuclear deterrence for Japan would be perceived by the other side as an increased threat, and both sides would fall into a cycle of anxiety, which would lead to a nuclear arms race. As a result, regional security in Northeast Asia could deteriorate. This is the risk of a nuclear arms race. This also suggests that efforts must be made to figure out how to prevent such a risk from turning into a security dilemma. Nuclear arms control bringing disarmament into sight is also one viable option in such an effort to avoid the negative spiral.
Third is the risk of undermining the international rule of law. Changes have been observed in international law relating to security. International treaties such as Additional Protocol I to the Geneva Conventions (AP I) 14 in 1977 and the Statute of the International Criminal Court (ICC Statute) 15 in 1998 were adopted since the NPT was adopted in 1968 during the Cold War.
Japan has been a party to the NPT since 1976. In the final document of the 2010 NPT Review Conference all the state parties agreed with "the unequivocal undertaking of the nuclear-weapon States to accomplish the total elimination of their nuclear arsenals leading to nuclear disarmament, to which all States parties are committed under article VI" (NPT Doc 2010, 19, A.ii.). It also affirmed that "all States" need to make "special efforts" to establish the "necessary framework to achieve and maintain a world without nuclear weapons" (NPT Doc 2010, 20, B.iii.). Japan, which has regarded the NPT as "the cornerstone of the international nuclear disarmament and non-proliferation" (Japan and the United States 2022), is expected to take the lead in efforts to eliminate nuclear weapons as the state that suffered atomic bombings in Hiroshima and Nagasaki.
Japan acceded to the AP I in 2004 and to the ICC Statute in 2007. There is international agreement that the "cardinal principles" of international humanitarian law apply to the use of nuclear weapons as "intransgressible principles of international customary law" (ICJ 1996, 257, paras. 78-79). A critical issue is whether these two international treaties, which are closely related to such cardinal principles, are compatible with Japan's security policy premised on US extended nuclear deterrence 16 The lack of compatibility could have a negative impact on international order based on rules and universal values including international humanitarian law and laws on international nuclear nonproliferation and disarmament. The NSS defined that one of Japan's national interests is to 14  "maintain and protect international order" based on "rules and universal values", such as freedom, democracy, respect for fundamental human rights, and the "rule of law" (Cabinet of Japan 2013a). Therefore, whether the sharing of roles which works toward solidifying US extended deterrence needs examination in light of the international law in order to pursue Japan's national interest.
The fourth is the consistency with the existing "Article 9 = Japan-US Security Treaty Regime [Kyujo = ampo taisei]" (Sakai 1991, 32). This framework was formed against the backdrop of the Cold War when the focus of the security challenge was deterrence. Today's focus has shifted to response, however, which has already impacted the "Article 9 = Japan-US Security Treaty Regime". The 2015 Peace and Security Legislation changed the interpretation of Article 9 of the Constitution to allow for a limited right of collective self-defense. At this point, one can already observe a modification of the regime. The more the focus of security issues shifts to response, the more the issue of the role of nuclear weapons in response comes to the fore. Although issues surrounding "Article 9 = Japan-US Security Treaty Regime" is beyond the scope of this article, the question is whether such a situation can be consistent with Japan's exclusively defense-oriented policy under the pacifism of Article 9 of the Constitution.

Conclusion
What is the qualitative change in Japan's security policy premised on US extended nuclear deterrence? It is that Japan's security policy has shifted from the mere stage of reliance to the stage of engaging with the United States in extended nuclear deterrence. This qualitative change works toward the solidifying of US extended nuclear deterrence for Japan, and it brings a greater risk of Japan being a target in the use of nuclear weapons in the changing security environment surrounding Japan. This entails the risks of creating new nuclear victims (hibakusha) and of further deterioration of the security environment in Northeast Asia due to the nuclear arms race. It is worth recalling the words of UN Secretary-General António Guterres: "Heightened tensions and dangers can only be resolved through serious political dialogue and negotiation -never by more arms. Disarmament and arms control measures can help ensure national and human security in the 21st Century, and must be an integral part of our collective security system" (UNODA 2021, iiv).
The ongoing solidification means that the sharing of roles between the two states in terms of security under the Japan-US Security Treaty is now extending to extended nuclear deterrence provided by the United States. As far as deliberations in the National Diet of Japan are concerned, however, there have been no examinations of the security risks associated with the ongoing shift, nor have there been any explanation of the security risks to the nation.
Japan upholds the "rule of law" as a national interest in the NSS and a pillar of its foreign policy (of Japan 2013a; Japan 2019b). Even with the severe security environment surrounding Japan, disregarding "universal values and rules" when formulating and implementing security policy would be contrary to Japan's national interests and incompatible with its foreign policy. Even if the Japanese government believes that extended nuclear deterrence is not about a normative argument over "right or wrong" (Motegi 2020, 7), it is difficult to believe that it denies that it is about a normative argument over "rules and universal values". Japan needs to abide by all international law relating to its security including the AP I, the ICC Statute and the NPT to respect the rule of law. The sustainability of Japan's security policy premised on US extended nuclear deterrence depends on the accountability of policymakers to the nation for these risks in light of national interests including the rule of law that Japan defined.

Disclosure Statement
No potential conflict of interest was reported by the author(s).

Notes on Contributor
Kimiaki Kawai is PhD candidate at the Graduate School of Global Humanities and Social Sciences, Nagasaki University. Kawai has been involved in activities for the abolition of nuclear weapons since the 1990s. Until October 2019, he served as director of the Soka Gakkai Peace Committee. He is a coordinator of the Japan NGO Network for Nuclear Weapons Abolition and a board member of the Japan Association of Disarmament Studies. Kawai works as a research fellow for the Toda Peace Institute and is also affiliated with the International Peace Research Institute, Meiji Gakuin University. Kawai holds an MA in political science from the Graduate School of International Politics, Economics and Communication, Aoyama Gakuin University (2020).