Not along after the war, it became clear to the United States that the Soviet Union was intent on reneging on the agreements made during Yalta concerning democracy in Eastern Europe. The Soviets already had boots on the ground in this part of the world, and were determined that they would dominate the Eastern Bloc to prevent a repeat of the kind of Western invasion they'd experienced during World War II. To that end, they subverted the political systems of countries such as Poland to make sure that they ended up with reliable puppet governments that they could control.

Stalin's imperialist policy towards Eastern Europe indicated to the United States that the USSR couldn't be trusted. The US and the Soviet Union had always been ideological enemies, but Stalin's strategic designs on East European territory provided an added dimension to the growing hostility between the two superpowers. The Americans were worried that the Soviets would take their campaign of subversion a stage further and set out to undermine the liberal democracies of Western Europe. This would represent a serious threat to the United States, both economically and strategically.

Most senior policy-makers in Washington agreed with George Kennan that the Soviet threat must be contained, and this inevitably meant that the United States would need to adopt an aggressive posture towards the USSR and the expansion of its sphere of influence.

The Cold War shaped much of world history over the next four
decades after the end of World War II. The Allies argued over the postwar settlement. The United States, Britain, and Russia met at the Teheran Conference in 1944. The decision for an invasion of France left Russia free to move into eastern Europe. The three met again at Yalta in 1945. The Soviet Union agreed to join against Japan in return for territorial gains in China and Japan. The United Nations was confirmed. Agreement over Europe's future was difficult. A disarmed Germany, purged of Nazi influence, was divided into four occupied zones. Eastern Europe, although promises were made for a democratic future, was left under Soviet domination.

The final postwar conference was at Potsdam in 1945. By then, the Soviets occupied eastern Europe and eastern Germany. They
annexed eastern Poland while the Poles gained compensation by receiving part of eastern Germany. Germany and Austria were divided and occupied. Japan was occupied by the United States and stripped of its wartime gains. Korea was freed, but was divided into United States and Soviet occupation zones. Asian colonies returned to their former rulers. China regained most of its territory, but civil strife continued between Communists and Nationalists.

In other regions, colonial holdings were confirmed. In Europe, Russia's frontiers were pushed westward to regain World War I losses. Most nations existing in 1918 were restored, although the Baltic states once again became Russian provinces. All except Greece and Yugoslavia fell under Soviet domination. Western nations were free, but under American influence.

The US and the Soviet Union became Cold War adversaries because they did not trust one another.  Each thought that the other side was going to try to take over the world.  For example, the US thought that the Soviets had taken Eastern Europe and put it behind an "iron curtain," thus going back on the promise of elections that it had made at Yalta.  The communists, meanwhile, felt that American policies like the Truman Doctrine and the Marshall Plan were meant to threaten them.  As they came to distrust one another more and more, they became enemies in the Cold War.