The growth of communism countries and the threat of a nuclear attack. They didn’t like Russia conquering so much land and felt capitalism. Also, they did not allow the people to vote for their government. The Communists were spreading rapidly. i.e. In 1949, Mao Zedong's communist forces had won a victory in China and this added another front to the Cold War. The blockade on Berlin set by Russia was another cautioning act. They were concerned when Russia launched the first successful spaceship called Sputnik into space. This set of the race to beat the other in outer space technology.
They felt threatened when they were invaded twice in the 20th century. Also, America had the Atomic Bomb and they were set to making their own. The western allies unified western Germany and did not consult Russia first. The Western Germany set up by the western allies had allowed them to re-arm and let them join NATO. America had been successfully testing bombs such as the hydrogen bombs and had moved them closer to Russia by moving into Russia. Also, they let America have partial governing of Berlin because they were afraid of the Atom bomb that was dropped in the same year. They were not ready to fight a war so they let America have part of Berlin which was deep in Russia's half of Germany after WW2.