Although the combat operations were over, the bloodshed was not. In 1940, at or near a place called Katyn, the Soviets executed 22,000 Poles, all of them unarmed, many of them civilians, and by that point in time, none of them part of the war effort.
This cold-blooded killing of civilians was conducted by the NKVD, one of the communist secret police agencies.
By 1941, the geopolitical landscape had changed. Hitler had betrayed Stalin, and Stalin had joined the western allies - England, the United States, and others.
The Allies were delighted to have the USSR on their side. They were hesitant to offend the Soviets in any way, lest they return to Hitler’s side.
President Roosevelt, hoping to spark a friendship with Stalin, did not confront him about the massacre at Katyn. Nor did FDR object to Stalin’s demand that, in a postwar scenario, almost half of Poland become Soviet territory: the USSR would annex 77,000 square kilometers of Polish territory.
Mentioning the killings at Katyn and the seizing of Polish territory, historians Stan Evans and Herbert Romerstein write:
Nor was that the total story. When the Red Army rolled back into Poland, the Soviets would control not merely half the nation but all of it. They would then set up a puppet regime in the city of Lublin for the part of the country still called “Poland,” plus a sector of Germany awarded the Poles in compensation for what was given Russia. To this further demarche the Americans and British consented with misgivings, but consent they did, covering their retreat with pro forma protests and never-to-be-honored Soviet pledges to provide for Polish free elections.
FDR, Winston Churchill, and Stalin met - occasionally with other global leaders - to make these deals and shape the postwar world in a series of conferences: in Teheran in 1944 and in Yalta in early 1945.
A conference was held in Potsdam later in 1945, but by that time, Roosevelt had died, and Harry S. Truman was president.
During his last two years in office, FDR was hampered in his ability to think and speak clearly. He was a dying man, diagnosed variously with cancer, hypertension, and heart failure. He often fell asleep at meetings.
In addition to his health, Roosevelt was also thinking about domestic politics inside the United States, as Stan Evans and Herbert Romerstein note:
Noteworthy in these events was the performance of FDR. One suggestive episode occurred at Teheran, when the President told Stalin the United States was willing to go along with the Soviets on Poland, but that he had political realities to deal with. “[T]here were six to seven million Americans of Polish extraction,” Roosevelt said, according to the official Teheran record, “and, as a practical man, he did not wish to lose their vote [in the 1944 election]. He hoped … that the Marshal would understand that, for the political reasons outlined above, he could not participate in any decision here in Teheran … and could not take any part in any such arrangement at the present time.” The magnanimous Stalin replied, now that FDR had explained it, that he understood the President’s problem.
Further inhibiting Roosevelt’s ability to negotiate was the fact that his advisor for foreign policy, Alger Hiss, was actually a Soviet agent, paid both to present a pro-Stalin view to FDR, and to leak American military secrets to Moscow.
Roosevelt hoped to form a working friendship with Stalin. Stalin had no intention of building any such relationship, but deliberately continued to foster those hopes in FDR, who was all the more inclined to allow the Soviets to continue their atrocities in Poland. Thus it was that millions of Poles lost their lives, their properties, and their liberty.