How did the Cold War between US and the Soviet Union grow out of World War 2?

Answer 1

At the Yalta Conference of 1945 the Western Allies had given Stalin the right to keep the countries that Russia had liberated from the Germans within his 'sphere of influence'.

Stalin's wish was only partly dictated by ideological motives such as 'the spread of World Communism'. Mostly it was dictated by Russian fear. Russia had sufferered innumerably larger losses than all the other Allies put together in WW 2. It did not even want to consider a renewal of the war in the form of a concerted action to "defeat" Communism: both Britain, France and the USA had actively fought the rise of Communism only two decades ago on Russian soil and several of the eastern liberated countries had been actively fighting Russia alongside Nazi Germany.

The Allies had conceded eastern Europe to the Soviets because at the time they were in dire need of Russia's armies to finish the job and do most of the battlefield dying for them. After 1945 they started sorely regretting their generosity, but not unnaturally Stalin held on to what he had been given. It did however quickly sour relationships, which were worsened further when Stalin (unsuccesfully) tried to force the Allies to give up their toehold of West Berlin in his newly Communist eastern Europe.

The clincher was Russia's development of an atomic bomb of their own, a thing which the US strongly supected could only have been done through espionnage within the USA on a massive scale. It was this last development which led to an unprecedented arms race in building litterally thousands of atomic missiles on both sides.

So finally it was the combination of traditional anti-Communist feelings in the West, resentment against Russia for holding on to the conquests they had been allowed in Yalta, and the subsequent arms race that led to the Cold War that was to end only in 1990.