This paper analyses Japan's security policy concerning the rising activism after the Cold War. In contrast to the traditional reluctance after World War II, Japan has become more and more willing to use its military power in the post-Cold War era. Today, Japan's Self Defence Forces (SDF) participate in peacekeeping operations and take a greater responsibility not only for East Asian regional stability but also more broadly for global security issues. Why has such a tremendous turn in the course of security policy been possible? Material considerations alone cannot lead us to convincing explanations for seemingly 'irrational' features of national security policy. Hence, the study employs a constructivist approach that pays attention to ideational factors as well as material calculations. Inter alia, it focuses on the 'military norm,' i. e. a social expectation of an appropriate state behaviour regarding the use of military power.I will argue that the rise of two positive (encouraging) military norms (the Liberal Order and the Self-Defence Norms) has overwhelmed the traditional negative (prohibiting) norm (the Anti-Military Norm) during the last fifteen years (1990-2004). After the Cold War, the Liberal Order Norm has risen as the most dominant one at the international level: it encourages states to use their military power to uphold the world order, anchored by liberal values (e. g. democracy and the market economy). It was first conveyed to Japan in the early 1990s through the Gulf Crisis/War, and subsequently led to the dispatch of the Japanese peacekeepers. From the mid-1990s onwards, the traditional Self-Defence Norm has also been revived, reflecting the remaining possibility of inter-state conflict in East Asia. The rise of the two positive norms first resulted in the SDF's greater responsibility for regional stability, and finally led the SDF to become more actively involved in global security affairs (e. g. the peacebuilding mission in the post-war Iraq). Thus, not only has the traditional reluctance receded but also a new activism has emerged in Japan, thereby motivating the nation to assume an increasingly active military role.