I propose to examine factors that have contributed to the maintenance of Japan-U.S. alliance and analyze the logic of its stability by weighing the “pillars” of the alliance at different levels of analysis, i.e., polarity (structure of the international system); polity (national goals and interests); and perceptions (political leaders). While the Japan-U.S. alliance is often described as one of the most stable bilateral security arrangements in today’s world, the nature of its stability is largely an unexplored area of study. With my longstanding interest in International Relations and Comparative Politics, I will investigate what constitutes continuity and change in the recently “redefined” alliance, drawing on insights from Political Science as well as case studies on decision-making processes in security policy. The central puzzle that runs through this project is the resilience of the Japan-U.S. alliance. In spite of the original rationale of the alliance disappearing after the end of the Cold War, the alliance itself has not only been maintained but even reinforced. Considering the rift that arose in such traditional alliances as NATO over the Iraqi issue, this stability constitutes an anomaly. While the nature of the alliance has been transformed to the extent that it cannot be called an alliance in an orthodox sense any more, why does it endure? How have the “pillars” of the alliance at different levels of analysis changed overtime? Most of the previous studies on alliance behavior have focused on the rather short-term policy agenda generated by the end of the Cold War. My objective is to frame the subject into a longer time span and into a larger spatial expanse in order to find the root factors that sustain the bilateral relations. In order to examine the weight of the distinct sources of stability at different phases of the evolution of the alliance since its formation, I will conduct longitudinal comparative case studies that entail a certain degree of readjustment of the relations between the two countries reflecting the changes in the international environment, national interests, and initiatives taken by leaders. The subjects will be: 1) negotiations up to the San Francisco Peace Treaty in 1951; 2) domestic decision-making and cross-national negotiations before Japan’s signing of the Non-Proliferation Treaty (NPT) in the 1960s and 1970s; and 3) the decision to cooperate in the Iraq War. These cases are expected to suggest that stability of the alliance can be explained by the dynamic equilibrium of the factors at different levels of analysis. By identifying the mechanisms for alliance durability, the study should help us obtain a clear idea of key forces for change and stability in managing future relations between Japan and the United States. The project is also distinctive in contributing to theories of alliance politics in general that can be applied to explain the behavior of other bilateral arrangements in the world. Further, the study should enhance our understanding of the security dynamics in Northeast Asia as the region of diversity and rapid change in the global society today.