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

According United States, Soviet Union wanted to spread the "disease" of Communism across the world, which would according to them create a new disorder in the world politics. The disagreements were felt even during the conferences, when the Allied leaders could not agree on certain things.

Explanation:

Wartime cooperation between two forces was mostly the product of them having the same enemy. So, when the end of war was approaching the disagreements between two sides were obvious. Soviets wanted a greater share in division of Germany, victory over Japan, etc., while on the other side United States disagreed with this, fearing the spreading of Communism.

d. islands off the coast of newfoundland