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Actually there was some concern over Stalin even before the war was over. Roosevelt could deal with him to some extent, but when he died and Harry Truman took office, I don't think he trusted Stalin at all. And at the end of the War, Germany and Berlin were divided into sectors for each of the victors, the US, Britain, France and the Soviet Union. Berlin, in the middle of East Germany was also divided into west, or US, British and French sectors and East, or Russian (Soviet). Stalin also took control of all of the parts of Europe he took when going into Germany, Poland, Czechoslovakia, Hungary, Romania, etc. While the western allies ruled West Germany for a time, and US troops are in Germany to this day with NATO, it was not an occupation force after about 1949. It was an occupation in East Germany until 1989
In 1949, the Soviet Union got the Atomic Bomb and that put it on a whole new level. Shooting wars in Korea and Viet Nam added to the tensions. The Berlin Crisis in August 1961 that led to the USSR and East Germany building the Berlin Wall brought great tension between east and west. The Cuban Missile Crisis in 1962 nearly ended in nuclear war. There were constant little wars in 3rd world countries with 'pawns' of the Russians and Americans fighting each other.