On 7 December 1941, the Japanese shockingly destroyed a large part of the American Pacific Navy. This attack on the USA, brought them almost instantly out of isolation and left the US fearful of possible further attacks, angry at the way they were attacked and they felt vulnerable due to the dramatic loss of many of the US Navy's fleet. From this point America was interventionist and was going to play a key role in world affairs and this can be seen as a cause of the Cold War.
During World War II, Russia and the U.S had always had to be allies and work closely together, in order to be successful in a common goal: the defeat of Nazi occupation throughout the war. At the end of the war, the U.S.A and U.S.S.R were clearly masters of the world. They were so far ahead of all the other countries in power and influence that they were called "superpowers".
As soon as the war ended, the relations between America and Russia cooled. American's regretted their isolation in the years between the wars. They now realised that they now had the power and resources to play a large part of world affairs. The lesson of the 1930's seemed to be that dictators must be faced and stopped, by force if necessary. Roosevelt had worked out America's aims during the war in the "Atlantic Charter". These included a United Nations (UN) Organisation to keep world peace, although Stalin had not taken part in these discussions. American's believed strongly in democracy and were deeply hostile to communism. As the war came to an end the differences between the Superpowers began to appear, especially when Stalin made it clear that for him, national security came from eliminating opponents and exerting his control over world communism.
Truman had been advised that the Soviet Union was behaving like a bully in Europe and should be told to mend its ways. Less than two weeks after taking office, Truman met with the Soviet Foreign minister, Vyacheslav Molotov at the White House.