This Info Flash examines the historical background to provide a comprehensive analysis of the US-Japan military cooperation. It delves into the current security situation shaping the Asia Pacific region, serving as the driving force behind the ongoing efforts to modernize and enhance military collaboration between the two nations. Despite their complex shared histories, the United States and Japan have significantly bolstered their military cooperation. The primary catalyst behind this strengthening of defence collaboration is the mounting instability within the Asia Pacific region. Japan confronts a multitude of challenges as the increasingly assertive China, which is pursuing a more aggressive regional security policy, and the persistent threat posed by North Korea’s missile and nuclear capabilities, including their missile launches targeting Japanese territory. Moreover, the full-scale invasion of Ukraine by Russia contributes to the volatility in the region. The resulting instability underscores the paramount importance of the alliance between the U.S. and Japan, emphasizing the need to modernize their forces and project their combined influence. This is crucial for promoting stability and upholding the rule of law in an international system that is growing increasingly turbulent.