This chapter provides an analysis of the political process that led to the unconstitutional informal constitutional amendment by the executive in Japan that lifted the ban on the exercise of collective self-defense through the government reinterpretation of Article 9 of the Constitution of Japan (Postwar Constitution) in July 2014 and the subsequent enactment of the security legislation in September 2015. Postwar Japan represents a curious case in which its postwar tradition of pacifism that is enshrined in Article 9 of the Constitution has been incrementally modified informally but remained formally unamendable politically for over 70 years for reasons that have nothing to do with court protection. It was not until the Shinzo Abe government’s lifting of the ban on the exercise of collective self-defense that the de facto constitutional amendment through the executive’s reinterpretation of the constitution was condemned so widely as unconstitutional, and indeed, has even revived a widespread opposition movement under the banner of constitutionalism against unrestrained, arbitrary rule.