What events caused Cold War between U.S. and the Soviet Union, who were allies during World War II?

The US and the Soviet Union were natural enemies. The US was a free market society that valued personal freedom and opportunity. The Soviet Union was a totalitarian dictatorship that insisted on state control of everything, and pushed enforced equality to the point of smothering any individual success.

They were only Allies in WW2 because they had a common enemy in Germany. The Cold War resulted from the huge difference in ideologies of the two powers and the paranoid rhetoric of the USSR. And the United States did not help the situation by taking the rhetoric at face value and making a big deal about the A-bomb, which the Russians feared would be used against them.

Plenty of great answers above mine. Both sides are equally to blame, but the Western Allies never quite understood how vulnerable the Soviets felt at the end of the war. The Soviets had been bled white in the conflict and even Truman at Potsdam noted how young the Soviet soldiers guarding the event appeared to be. Stalin was certainly aggressive in his desire to create an Eastern Bloc of states under Soviet control, which would act as a buffer to any Western attacks. The dude was paranoid and after 4 brutal years fighting the Germans and their allies, which included: Finns, Romanians, Hungarians, Bulgarians, Slovaks, as well as foreign legions made up of French, Danes, Norwegians, Dutch, and Spanish troops, it would be difficult not to be a little bit paranoid. And the rearming of West German police and then military sent alarm bells ringing in Stalin's mind. The 1948 blockade was a direct response to his paranoia. It is easy for us in the West to forget how the Soviets viewed us from 1941 on. They truly believed that the Western Allies slow-blinked them on the Second Front, while Hitler's armies were slowly ground down in Russia and the Ukraine. And of course Stalin's paranoia was shared by the vast majority of Americans and Western Europeans who were convinced that the Soviet Union stood ready to launch a campaign to spread communism to all corners of the globe.

rohak's answer was pretty accurate. Ever since the 15th century it's been a major goal of European (and American as a result) society to make a balance of power within Europe, always trying to have 3 or 4 major powers. Sometimes it got out of hand, like in the Napoleanic era. That is the only major thing I'd add to rohak's answer, is that if the U.K had been as power and controlled as many resources as the USSR did before its collapse, the relationship between the U.K. and the rest of the Western powers would have been extremely strained as well.

You will get a thorough answer with this link.

wiki.answers.com/Q/What_caused_the_Cold_War