Thucydides has been considered as the father of the realist theory of international politics. Most realists believe that the phrase in the Melian dialogue in his History of the Peloponnesian War—“The strong do what they can and the weak must suffer they must”—is the essence of realist theory and was Thucydides' own position.
This article tries to rectify such a view of Thucydides. I shall present a more nuanced view of his History based on the studies of the late Professor Michitaro Tanaka, the expert on Greek philosophy in Japan, and Professor Masaaki Kubo, who is one of the best Japanese translators of Thucydides, as well as on recent researches which have appeared mostly after the end of the Cold War, conducted by international relations specialists.
Special attention is paid to four incidents in History, namely, the Mytilenaean debate, the Pylos incident, the Melian dialogue, and the Sicilian expedition. Between the incidents at Pylos and Melos, I shall argue that Athens gradually lost prudence, and became “fundamentalist”, to use Michael W. Doyle's word. At Melos, Athens reached “the last level of intellectual sclerosis” (Kubo), and the result of sclerosis was the tragedy of the Sicilian expedition in which Athens lost more than 60, 000 soldiers, including Demosthenes and Nicias. Then, the question is why Athenians lost their realistic eyes in international affairs. I shall present two answers: from a short term perspective, the accidental success at Pylos led Athens to expect more success to follow, and from a long term perspective, the fear of the consequences of loss led Athens' leaders to take overly ambitions actions.
Considering the History in this regard, I believe that Thucydides was critical of Athens' behavior at Melos, contrary to the view conventionally held. In the end, I identify Thucydides with the line related to “defensive realism, ” i. e., the Thucydides-Rousseau-M. Wight/J. Herz-R. Jervis/R. N. Lebow line, not the Thucydides-Machiavelli-Hobbes-Morgenthau-K. Waltz line. I find that in the History his logic went well beyond realism. Though I still believe that realism is going to remain in the mainstream of international relations, the logic of realism in the 21st century should be reconstructed in line with the “Thucydidian realism” presented in this article.