Because the Soviet Union wanted to spread communism, but the Americans did not want that so they stopped being the Soviet Union's ally.
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The high level of America's wartime production was one of the main factors.
There were tons of alliances made that caused ww1. Alliances made countries stronger.
secret alliances
Mutual Defense Alliances. Imperialism.Militarism. Nationalism and I dunno the other.
Much like in WW1, there were 2 alliances: The Allies which consisted of Britain, French evacuees, Canadians, Austrailians, Indians (real India), and later the USSR and USA, and The Axis which consisted of Germany, Italy, Japan, and many other loyal countries. For a short time, the USSR was neither Allies nor Axis. Germany and the Soviets signed a peace agreement: You don't kill me and I won't kill you. The agreement was over with the attempted Russian assault.