After the Second World War, the United States emerged, along with the Soviet Union, as one of the world's superpowers. With Germany and Japan in ruins, and Great Britain having expended all of its resources to fight the war against Germany, the United States, removed from the carnage and destruction directly, gained great geopolitical as well as economic clout. Politically, the United States and the Soviet Union clashed over the future of Eastern Europe. The Soviets under Stalin wished to create a buffer zone of communist states between its borders and Western Europe. But after receiving false promises from Stalin that democratic elections would be allowed in Poland, the United States interpreted the expansion of communism into Eastern Europe as aggression that had to be met with firmness. So by the late 1940s, Europe was divided between a communist bloc of states under Soviet influence and the West, which was heavily influenced by the United States.
The economic role of the United States was itself shaped by this situation. The United States economy boomed during the war, as it expanded to meet the needs of its own people and armed forces as well as that of the Allies. American bankers as well as the federal government extended massive loans to American allies, and shortly after the war's end, the US government poured billions of aid into Western Europe in the form of the Marshall Plan. American manufactured goods duly poured into Western Europe as well as Japan (especially with the onset of the Korean War) and the American economy, far from experiencing a postwar depression predicted by many observers, actually expanded dramatically during the 1950s.
The United States exercised its powers through the institutions designed to establish a new global postwar order--the United Nations, NATO (established in the late 1940s) and the International Monetary Fund. The United States had been expanding throughout the twentieth century, but World War II was the pivotal event in establishing the nation as a global superpower whose influence was rivaled only by its Cold War adversary the Soviet Union. The isolationism of the prewar era was a thing of the past, and the United States was inextricably entwined with events around the world.