Japan’s evolving security policy has attracted considerable policymaking and academic theoretical debate and carries major implications for future regional stability. This chapter, in tracing the development of Japanese “Grand Strategy,” national defense capabilities, and the US-Japan alliance, challenges many of the key existing paradigms. It argues that, contrary to conventional policy and academic wisdom, Japan is continuing along a trajectory of becoming a more assertive military power in the defense of its own territory and in support of US military strategy in East Asia and globally. Japan is driven along this path predominantly by international structural pressures that are overcoming, and, indeed, triggering declining domestic resistance to this agenda. Japan’s harder-edged military role is questionable as a force for stability in the region; more likely are security collisions with China and neighbors, and even frictions with the United States.