This chapter analyses the factors driving Japan to expand the Self-Defence Force’s (SDF) security role within the framework of the alliance with the United States (US). The chapter investigates three cases: the redefinition of the alliance with the US in 1996, Japan’s support for the US-led war in Afghanistan started in 2001, and the US-led Iraq War started in 2003. In the 1990s, Japan enlarged the SDF’s role beyond self-defence by allowing it to support US military action in areas surrounding Japan. It then expanded these roles by adopting two special bills designed to assist the US-led ‘War on Terrorism’ and the Iraq War. By investigating the distribution of ideas on the political front along with the international norm, the chapter argues that these combined factors encouraged Japan to enlarge its military roles.