At the end of WWII, why was the fate of Poland so important to U.S. and the Soviet Union?
The fate of Poland was important to the US and the USSR because of the looming Cold War.
By the end of WWII, it was clear that relations between the US and the Soviets would not be very friendly. Because of this, Poland became an important issue. The Soviets wanted to control Poland for at least two reasons. Partly, they wanted to spread communism. In addition, they wanted a buffer between them and the West to help protect them from invasion.
The US, by contrast, was suspicious of the Soviet Union. It felt that Poland should be democratic. It wanted this partly because it wanted to prevent the Soviets from getting too much power. It also felt that Poland had been abused by the major powers before WWII and it wanted to treat Poland more fairly.