The war against Japan (1941–1945) gave rise to a uniquely enduring alliance between the United States, Australia, and New Zealand. Rooted in overlapping geopolitical interests and shared Western traditions, tripartite relationships forged in the struggles against fascism in World War II deepened as Cold War conflicts erupted in East and Southeast Asia. War in Korea drew the three Pacific democracies into a formal alliance, ANZUS. In the aftermath of defeat in Vietnam, however, American hegemony confronted new challenges, regionally and globally. A more fluid geopolitical environment replaced the alliance certainties of the early Cold War. ANZUS splintered but was not permanently broken. Thus the ebb and flow of tripartite relationships from the attack on Pearl Harbor to the first decades of the “Pacific Century” shifted as the “war on terror” and, in a very different way, the “rise of China,” revitalized trilateral cooperation and resuscitated the ANZUS agreement.