Why was world war ii followed so quickly by the cold war ? (p.936)?

During World War II the United States and the Soviet Union fought as allies against the Axis Powers. But there was always a sense of distrust and hostility between the two countries. The Americans were afraid the Soviet Union planned to take over the world and spread their communist ideologies. The Soviets begrudged the Americans for refusing to accept the USSR as a legitimate part of the international community. Then an arms race began between the two countries. The hydrogen bomb was built. The rivalry extended into space when the Soviets launched the first satellite, the Sputnik. Then they launched the first man into space in April 1961. On July 20, 1969 Neil Armstrong became the first man to step on the moon, thus making the United States the winner in the space race. In June 1950 the Soviet backed North Korea invaded the pro-Western South Korea. Troops were sent to Korea but the war did not end until 1953. The Cold War finally ended in 1991 when the Berlin Wall was torn down and the USSR collapsed. They no longer had influence in Eastern Europe.