The purpose of this dissertation is to examine the policy of defense of Japan in a different context from that stimulated their formation at the end of World War II. For this goal, I intend to make a historical study on the regional security environment of Japan and its consequences on institutional reforms in the field of national security in Japan. The factors selected are distributed in the systemic, regional and domestic levels. The historical cutoff of this work is the period of the post-Cold War, when the fragmentation of the threats turned regional security in East Asia. The theoretical guidance adopts the neoclassical realism to a broader understanding on the perceptions of threats. To this end, I analyze the case of changes in defense policy in Japan using the theory of the balance of threats developed by Stephen Walt specifically applied in both cases leading to perceptions of threats power form the part of Japan: The instability in the Korean peninsula and economic and military growth of China. The main hypothesis of this work suggests that the changes in the strategic environment in East Asia after the Cold War allied to dissatisfaction with the answers given by the Japanese foreign policy in times of crisis have given new life to supporters in Japan as "Normal State." The increase in perceptions of the threats from the part of Japanese leaders has changed the pacifist niponic stance and Japan started to act more assertively in security matters.