This chapter examines Japan’s peacekeeping policy. It studies the factors that prevented Japan from participating in United Nations Peacekeeping Operations (UNPKO) throughout the Cold War period. At the time, domestic politics was severely divided over the interpretation of Article 9 of the Constitution, with the left-wing parties arguing that the Self-Defence Forces (SDF) and the Security Treaty with the United States were unconstitutional. However, the Liberal Democratic Party government considered that such a participation would not violate the Constitution if the SDF refrained from using force. However, the government chose not to send the SDF to avoid provoking disputes over any such dispatch. In fact, Japan was not the only country that did not participate in UNPKOs during the Cold War. The chapter illuminates how the political competition emanating from different ideas and approaches to national security constrained Japan’s policy.