The introductory chapter provides an overview of the background to the study. It explains the aims of the book, my central argument, and the contents of the book. Japan’s security policy has attracted wide scholarly attention because during the Cold War, its policy was an anomaly while in the post-Cold War era, its transformation has been remarkable. Taking a constructivist approach that emphasises the constitutive and regulative effects of norms, this book argues that two factors frame Japan’s security policy: one is the distribution of collective ideas regarding the interpretation of the domestic norm among political parties, the other is an international norm that entered the society and was then localised. While these factors affect Japan’s policy-making process, its security policy is eventually formed within a pacifist tradition.