What made it so difficult for the united states to reject the demands of joseph stalin for establishing a soviet sphere in eastern europe?

The reason why it is so difficult for the united states to reject the demands of joseph stalin for establishing a soviet sphere in eastern europe was: Roosevelt realized the sacrifices the Soviets had made in their victory in the eastern front

Before the cold war, United States and the Soviet Union were under the same coalition to defeat Hitler and his forces.
The Soviet union has proven themselves to be a reliable ally by stopping the majority of Hitler's movement in Eastern Europe.
Because of this, United States fell inclined to gain favors made by the soviet union, including the demand for soviet sphere.