This article analyzes a broad range of U.S. and Japanese perspectives on Japan’s defense buildup in the 1970s, a buildup that simultaneously showed assertiveness and caution. President Richard M. Nixon announced what soon became known as the Nixon Doctrine, under which the United States began pressuring Japan to take on greater defense and regional responsibilities. U.S. officials assessed the extent to which Japan’s security role might affect U.S.-Japan burden sharing within the quadrilateral interactions in East Asia involving the United States, the Soviet Union, China, and Japan. Japanese leaders considered whether Japan should develop an autonomous defense posture (including the controversial question of Japan’s nuclear weapons program) or should seek qualitative improvements of its conventional self-defense force capabilities (within the context of its domestic institutional and normative restraints). The article concludes by assessing the political-security implications of the institutionalization of U.S.-Japan defense cooperation.