Straightforward realist calculations of political and economic interests would suggest much higher levels of cooperation than have been evidenced between the Republic of Korea (ROK) and Japan. An often-cited example of their puzzling interaction is the June 1965 normalization treaty. While highly beneficial to both countries, this settlement came about only after 14 years of acerbic and seemingly unnecessarily protracted negotiations. Conventional wisdom highlights the historical enmity and psychological barriers that separate Koreans and Japanese as a key explanatory variable for the tortured treaty process as well as the primary impediment to generally greater cooperation in the relationship. However, recently declassified U.S. government documents and new primary source materials shed new light on the importance of broader geostrategic variables in the 1965 settlement. This article contends that the heightened Cold War security environment in East Asia compelled a normalization settlement between Korea and Japan. The primary causal factor in this regard was the United States. Rising threats from China, coupled with burdensome entanglements in Vietnam, prompted the U.S. to place the highest priority on reconciliation between its two key allies in the region. While U.S. pressure for an ROK—Japan settlement had existed since the start of negotiations in 1951, these efforts became particularly pronounced from 1964 and were critical to the treaty's conclusion in 1965. These findings have implications for the study of Korea—Japan relations generally as well as for the application of international relations theory to East Asia.