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.