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.

Why did the so-called hub-and-spokes alliance system emerge in East Asia after World War II instead of a multilateral alliance? Realists and constructivists offer various explanations, pointing to such factors as the United States' preference for bilateral alliances, the absence of a collective identity, and historical memories of Japanese imperialism. None of these explanations is satisfactory, however. Indeed, the historical record reveals that the United States sought a multilateral alliance in East Asia until the early 1960s. A theoretical model based on a social exchange network approach explains how a specific form of network can develop among potential allies. In East Asia, three U.S. allies—Japan, South Korea, and Taiwan—contributed to the emergence and shape of the hub-and-spokes system, which came into being as an unintended consequence of their interactions. The preferences and behavior of these allies proved at least as consequential as those of the United States in shaping this system. The implications of this finding could be significant for alliance politics in contemporary East Asia.

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