Since the end of the Cold War, the most threat to international peace and security, not from a dispute or conflict between countries, but a humanitarian catastrophe within a country caused by ethnic conflict, civil war or state failure. The New Interventionism is to break the principle of non-interference in the internal affairs, and by use of force to address issues such as Bosnia, Rwanda, Kosovo, East Timor, Sudan, Congo, Haiti and other humanitarian catastrophes. According to the reports of United Nations Secretary-General and the resolutions of United Nations Secretary Council, as well as scholars’ researches, the New Interventionism has become a security strategy to prevent and resolve conflicts, as well as to reconstruct in post-conflict within a country. In June 1999 the Cologne EU Council decided to develop an autonomous capacity to launch the EU Common Security and Defence Policy, thus giving EU as full role on the international stage. In 2003, EU launched its first military intervention in the Former Yugoslav Republic of Macedonia. This dissertation discusses the combination of the above two, through the New Interventionism as a research approach of validating EU's military intervention. The policy of EU military intervention is not collective defense in essence, but one for the maintenance of international peace and security in the New Interventionism. Through these five cases: Operation Concordia in FYROM, EUFOR Althea in Bosnia, Artemis in Congo, EUFOR RD Congo and EUFOR Tchad / RCA, this dissertation will validate the New Interventionism in theory and practice and evaluate the results and capacity of the EU's military operations, and explore whether EU can be an effective, stable and creditable peace-keeping force. The New Interventionism is still the best idea to deal with a country’s internal crisis continuing to constitute a threat to international peace and security, even if it could be abused. And EU’s military intervention will be legal as a precondition, especially under the authority of UN, although the New Interventionism tolerates use of force based on legitimacy. Because of the strategic culture and preferences of the Member States, in the foreseeable future, EU is hardly to develop the same military capabilities as NATO's; the peace-enforcing operation will not be the EU's option. EU will go toward two modes in military intervention: to take over NATO's mission under Berlin Arrangement, and support the United Nations peacekeeping force.