Just like Reagan and Thatcher, and Churchill
An unexplained phenomenon: why would Churchill favor going to war to get Nazis out of Poland,yet meekly acquiesce when communists occupied the country after 1944-45? I've never understood the answer to that question. Nazis and communists were different forms of socialism, which Churchill was said to have decried.
Because Soviet troops were standing on Polish soil, Stalin had a very strong negotiating position at Yalta. Roosevelt had a distrust of colonialism and although he personally liked WSC, he was wary that the Prime Minister might be putting British interests ahead of the aims of the Allies. So he didn't support Churchill's demands for a well defined agreement on Poland.
In the end Roosevelt & Churchill settled on a vague plan for a provisional government for Poland to be followed by free elections. Stalin packed the provisional Lublin government with pro-USSR communists that refused to allow London's Polish government in exile to return and form a political party for the elections.
By the time of the Potsdam conference, Roosevelt was dead, but Truman followed FDR's plan and didn't want to challenge Stalin even though it was clear that "Uncle Joe" had lied and intended to dominate its neighbors. Short of attacking a fully mobilized Soviet Union there was not much the US/UK could do. A week later Churchill lost the election and he left office rather bitter about Stalin's treachery and Truman's weak showing.