Since the late 1990s, Japan has sent increasing numbers of its military forces overseas. It has also assumed a more active military role in the U.S.-Japan alliance. Neither conventional constructivist nor realist approaches in international relations theory can adequately explain these changes or, more generally, changes in Japan’s security policy since the end of World War II. Instead, Japan’s postwar security policy has been driven by the country’s powerful antimilitarism, which reflects the following normative and realist factors: pacifism, antitraditionalism, and fear of entrapment. An understanding of the influence of these three factors makes it possible to explain both Japan’s past reluctance to play a military role overseas and its increasing activism over the last decade. Four case studies—the revision of the U.S.-Japan Security Treaty in 1960, the anti–Vietnam War period, increases in U.S.-Japan military cooperation during détente, and actions taken during the administration of Junichiro Koizumi to enhance Japan’s security profile—illustrate the role of antimilitarism in Japan’s security policy. Only through a theoretical approach based on analytical eclecticism—a research strategy that considers factors from different paradigms—can scholars explain specific puzzles in international politics.