Focusing on the internal security of Japan, this paper reconsiders the origins of the US-Japan alliance, a central arrangement in Japan's peace settlement of 1951. Existing studies maintain that concerns about military attacks by communist states against Japan constituted the chief rationale behind the alliance. In contrast, this paper argues that the fear for communist revolutions inside Japan played a more significant role in its formation. Indeed, much of the US policy towards Japan in the early Cold War was designed specifically to contain the spread of communism in Japan. Moreover, while the outbreak of communist revolutions had been considered a long-term security threat by Japanese policymakers since the 1920s, it became a major preoccupation of the Yoshida administration under the Allied occupation. By discussing how both Japan and the US sought to radicate democracy and to prevent communist revolutions in Japan, this paper analyzes the decision-making processes us to September 1950 when the US government decided to ally with Japan and to station US military forces in the country after the peace. This paper highlights the interaction between post-war US-Japan relations and the development of Japan as a democratic society while reassessing the role of the US-Japan alliance.