This chapter looks at the grand strategic implications of the Japanese invasion of Korea in the 1590s, which was successfully thwarted by a Sino-Korean alliance that emerged out of China’s obligations to Korea as part of the so-called tributary system of foreign relations. The Great East Asian War of 1592–1598, known to Koreans as the Imjin War, was the largest conflict on the globe in the sixteenth century yet it is still barely known outside of East Asia. The chapter will offer an overview of how the war fit into the ongoing grand strategy of Ming-dynasty China as it sought to preserve its hegemonic position in East Asia. It will also examine the motivations and strategic calculations of the Japanese hegemon, Toyotomi Hideyoshi (1536–1598), who sought to overturn the longstanding Ming order and create a new international system that could have fundamentally altered the course of Asian, if not world, history had it succeeded. In the end, the defeat of Hideyoshi’s ambitions preserved the East Asian world order and China’s hegemonic position therein for another 250 years. In addition to examining the motivations and ramifications of the war through primary research, this chapter touches upon some of the recent historical and political science literature concerning the war and its broader implications for the study of international relations and power politics in the early modern East Asian context.