The U.S. emerged from World War II the world's richest and most militarily powerful nation. As a liberal democracy that had just defeated fascist dictatorships in two theaters, the country – if not all of its people – felt a sense of exceptionalism and historical purpose. So did the Soviet Union, its ally in the fight against Germany and the world's only revolutionary Marxist state. The two powers promptly divided Europe into spheres of political and economic influence: Winston Churchill called this dividing line the "Iron Curtain."

The two superpowers, both of which possessed nuclear weapons after 1949, engaged in a long standoff known as the Cold War. Due to the doctrine of Mutually Assured Destruction – the belief that a war between the two powers would lead to a nuclear holocaust – no direct military engagements occurred between the U.S. and the Soviet Union, and the Iron Curtain was largely quiet. Instead they fought a global proxy war, with each sponsoring friendly regimes in post-colonial nations in Africa, Asia and Latin America. The U.S. and Soviet Union both sponsored coups to install such regimes in various countries.

The closest the U.S. came to a direct military conflict with the Soviet Union was the 1962 Cuban missile crisis. The U.S. did fight a prolonged hot war in Vietnam, however, in which its military supported South Vietnamese forces fighting the Chinese- and Soviet-supported North Vietnamese army and South Vietnamese communist guerrillas. The U.S. withdrew from the war and Vietnam was united under communist rule in 1975.

The Cold War ended with the collapse of the Soviet Union in 1991.