Why did the U.S. get involved in the Cold War?

At the end of World War II, Germany and Japan were destroyed, Britain was broke and France was unstable and preoccupied in French Indochina.  The Soviet Union and the United States were the only countries/empires left on the planet.  We had opposite systems of government, and opposite desires for the postwar world.

The only thing that prevented the Cold War by 1943 was our alliance against Nazi Germany.  Once the war was over, there was little reason for us to be friends, and a mutual nuclear threat to keep us on our toes.  Given this environment and history, the Cold War was inevitable.