The relationship between the USSR and the USA post WWII became very fragile. During WWII the Soviet leader (Stalin) refused to join the United Nations which brought about tension. The Soviet Union was a communist country and U.S wanted to spread democracy and strongly opposed communism. The Soviet's tried to take over part of Germany while the UN was trying to rebuild it. In the end, all of these tensions rose so high it caused the Cold War, the war between the Soviet Union and the U.S.A.