Why was there tension between the united states and the soviet union from 1945 until 1991?

The Cold War began as World War II was ending. American leaders saw the power and ambitions of the Soviet Union as a threat to our national security. The Cold War was a war of words and ideologies rather than a shooting war, although at times the Cold War turned "hot" as in Korea and Vietnam. Basically, the Cold War was a rivalry between the United States as leader of the western democracies, and the Soviet Union and the nations that were controlled by the communists.