From the American perspective, why did wartime cooperation between the United States and the Soviet Union collapse in 1945–46?

During 1945 after the Potsdam Conference one of the main question was division of Germany. According to agreement Germany was to be divided into four separate zones. This was unacceptable for Soviets, who wanted a bigger peace of cookie, but United States didn't wanted to allow this, as they were afraid that it will affect the world order.

Explanation:

During WW2 it was obvious that ideological differences between Soviet Union and United States can create a problem for the whole world. They couldn't totally agree how the world should look like after the war. The problems started with the division of Germany, but continued in the other areas of the world also.

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