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Paper short abstract

This study traces the origins of U.S.-Japan defense cooperation by showing how the two countries began joint military planning in the early 1950s. Examining the actual practice of defense cooperation under domestic constraints, it reevaluates the U.S.-Japan security relationship during the Cold War.

Paper long abstract

In the wake of Russia’s invasion of Ukraine and growing concerns about a potential contingency involving Taiwan, the role of the U.S.–Japan alliance and specifically collective defense in a contingency—has taken on unprecedented importance. This study explores the historical origins of bilateral defense cooperation that are directly relevant to this contemporary concern.

It shows how the United States and Japan began to develop combined military plans in the early 1950s. The study clarifies points that existing works have not fully explained: why Japan, initially reluctant toward the formulation of these plans, later re-engaged in the planning process, and what negotiations took place behind the scenes. During these negotiations, the most divisive issue was the command relationship between the two forces—what kinds of command structures were considered between the United States and Japan? The study also demonstrates that, in the early 1950s, Japan sought approval of these plans by the highest governmental authorities in both countries. Furthermore, it illuminates the challenges faced by both the United States and Japan in balancing the military necessity of undertaking joint defense measures against the Soviet Union and other communist forces with the constraints of their respective domestic politics. These findings challenge the prevailing view that the plans were merely staff-level research by the two forces and instead reveal the early origins of collective defense in the U.S.–Japan alliance.

This study draws on extensive multi-archival research, particularly newly declassified materials from both U.S. and Japanese sources. By uncovering the actual practice of U.S.–Japan defense cooperation under domestic constraints, it offers a re-evaluation of the Cold War U.S.–Japan security relationship. Moreover, this study distinguishes itself from conventional scholarship by emphasizing not only the political and diplomatic dimensions of the U.S.–Japan security relationship but also its military and, crucially, operational dimension—an aspect that has been largely overlooked. This approach offers novel insights into the fundamental nature of the U.S.–Japan security relationship.