The single most important feature of American history after 1945 was the United States’s assumption of hegemonic leadership. Europeans had noted America's enormous potential since at least the nineteenth century. After the Civil War the United States had one of the largest economies in the world, but, as noted earlier in this book, in geopolitical terms it remained a surprisingly minor player.1 By 1900 the United States was playing a more significant political role. But it was only after 1945 that the nation's potential on the world stage was fully realized. Victory in the Second World War left the United States in an enviable position. Unlike the Soviet Union, which endured devastating fighting on its territory and lost tens of millions of citizens, the United States had experienced only one major attack on its soil. Thanks to its actions in the war America had great influence in Europe. And the national economy emerged surprisingly vibrant from the years of conflagration, easily dominant over any conceivable rival or set of rivals.
When the First World War ended the United States ultimately chose to return to its hemispheric perch. It declined to join the new League of Nations, and rather than maintaining engagement with the great powers of the day, America generally turned inward.2 The years following the Second World War were quite different. In addition to championing—and hosting—the new United Nations, the United States quickly established a panoply of important institutions aimed at maintaining and organizing international cooperation in both economic and security affairs.3 Rising tensions with the Soviet Union, apparent to many shortly after the war's end, led the United States to remain militarily active in both Europe and Asia. The intensifying Cold War cemented this unprecedented approach to world politics.