Constructivists have advanced the study of Japanese national security policy by illuminating how normative factors shape state behavior. At the same time, they have overemphasized the role norms and ideas play while downplaying the structural and material forces that often underlie normative factors. This article seeks to reveal the political foundations of Japanese postwar pacifism. It maintains that explanations based on norms and identities cannot be separated from discussion on material and structural factors when it comes to the question of where norms come from and why they are sustained. Power and interests may not explain everything, but they often account for why certain norms emerge and are sustained to influence policy. By examining the shifts in public opinions and the Social Democratic Party's defense policy, this article argues that Japan's postwar pacifism has been possible in large part because peace was relatively abundant in postwar Japan and that the majority of the Japanese felt that the alliance with the United States contributed to that effect.