By 1941, the USSR would join the western allies and fight against Germany. But the Soviet change in alliances did not change Soviet behavior toward the Poles.
In 1940, a unit of the NKVD, part of the Soviet secret police, executed approximately 22,000 Poles at or near a place called Katyn. The victims were unarmed, mostly civilians, and not part of any combat operation: the invasion of Poland had already ended in late 1939.
In addition to terrorizing the Poles, Stalin also annexed Polish territory. In 1945, the USSR seized 77,000 square kilometers of Poland and made it part of the Soviet Union.
The question raises itself: why did the western allies allow, or even enable, the USSR to perpetrate atrocities against Poland? Historians Herbert Romerstein and Stan Evans write:
Among the most striking features of wartime diplomatic history was the oft-repeated belief of the Western leaders that they had to make concessions to Moscow, while asking little or nothing in return from Stalin. The rationale for this would vary from one case to the next: to keep the Soviets from making a separate peace with Hitler, to build up their confidence in our intentions, to reward them for “killing the most Germans,” to placate them because of their great military power. Whatever the stated purpose, the result in nearly all such instances was the same: to give Stalin things that he demanded.
The postwar world was shaped by a series of agreements made among the Allies at a series of major conferences. These meetings were held at Teheran in 1944, and at Yalta and Potsdam in 1945.
Stalin claimed that he needed to keep Poland under Soviet domination because it would act as a defensive shield: a “buffer” zone. Anyone trying to invade the USSR by going through Poland, Stalin argued, would be stopped before reaching Russian soil.
In reality, however, no such potential aggressors existed by the time Stalin used this excuse: Germany and Italy had been soundly defeated, and their industrial and military strength dismantled.
The scenes at Teheran and Yalta where these matters were discussed would read like a comedy of errors if they hadn’t been so tragic. Stalin’s posturing on the danger of invasion via Poland — a country he had himself invaded — has been noted. Equally bizarre were his objections to having outside observers monitor Polish elections, on the grounds that this would be offensive to the independent-minded Poles. This was said by Stalin with a presumably straight face, even as his agents were imposing a brutal dictatorship in Poland that would crush all hope of independence. All this was known by the Western allies to be bogus, but in the end they would swallow the whole concoction.
One reason why the western allies stood by while the USSR ravaged Poland was because the NKVD had placed secret agents (“moles”) inside their governments. These Soviet operatives fed only selected bits of information to the policymakers in the allied governments.
Winston Churchill was one of the few western allies who was alert to Stalin’s deceptions and plans to create a communist hegemony in eastern Europe. Churchill was, however, unable to alert or persuade other western leaders about this.
Allied foreign policy was therefore shaped on the basis of reports generated by the international communist conspiracy. Acting on the Stalinist propaganda which had been presented to them as military intelligence or foreign policy analysis, the Allies let the USSR occupy and oppress Poland under a ruthless dictatorship for several decades after 1945.