Over the past 150 years, the United States and Japan have developed one of the United States’ most significant international relationships, marked by a potent mix of cooperation and rivalry. After a devastating war, these two states built a lasting alliance that stands at the center of US diplomacy, security, and economic policy in the Pacific and beyond. Yet this relationship is not simply the product of economic or strategic calculations. Japan has repeatedly shaped American understandings of empire, hegemony, race, democracy, and globalization, because these two states have often developed in remarkable parallel with one another. From the edges of the international order in the 1850s and 1860s, both entered a period of intense state-building at home and imperial expansion abroad in the late 19th and early 20th centuries. These imperial ambitions violently collided in the 1940s in an epic contest to determine the Pacific geopolitical order. After its victory in World War II, the United States embarked on an unprecedented occupation designed to transform Japan into a stable and internationally cooperative democracy. The two countries also forged a diplomatic and security alliance that offered crucial logistical, political, and economic support to the United States’ Cold War quest to prevent the spread of communism. In the 1970s and 1980s, Japan’s rise as the globe’s second-largest economy caused significant tension in this relationship and forced Americans to confront the changing nature of national power and economic growth in a globalizing world. However, in recent decades, rising tensions in the Asia-Pacific have served to focus this alliance on the construction of a stable trans-Pacific economic and geopolitical order.