On the eve of a major offensive into Poland, the Soviet Union decides to recognise the pro-Soviet Lublin Committee as the Provisional Government of Poland instead of the government-in-exile that was temporarily being headquartered in London.
On September 1, 1939, a massive German army invaded Poland. Sixteen days later, the USSR invaded Poland from the east. During this tumultuous period, Gen. Wladyslaw Sikorski became leader of a Polish government-in-exile in London. He developed a good working relationship with the Allies until April 1943, when Soviet Premier Joseph Stalin broke off Polish-Soviet diplomatic relations after Sikorski requested that the Red Cross investigate the alleged Soviet slaughter of Polish officers in the Katyn forest of eastern Poland in 1942.
As the war progressed and the Soviets battled the Germans in western Poland, the Polish government-in-exile began to fear that Soviet domination might follow if the Soviets defeated Germany for control of the Polish territory. Stanislaw Mikolajczyk, Sikorski's successor as the provisional government head, pleaded with the Allies to secure Poland's postwar borders and sovereignty, but no such assurances were granted. In August 1944, the Polish Home Army, fearful that the Soviets would march on Warsaw to battle the Germans and never leave the capital, led an uprising against the German occupiers. They hoped that if they could defeat the Germans, the Allies would help install the anti-Communist government-in-exile after the war.
Unfortunately, the Soviets, rather than aiding the uprising that they encouraged in the name of beating back their common enemy, stood idly by and watched as the Germans slaughtered the Poles and sent survivors to concentration camps.
With native Polish resistance eradicated, and in anticipation of one last offensive against the Germans, the Soviet Union created its own pro-communist Polish provisional government to counter the anti-communist government-in-exile. At the Yalta Conference in February 1945, the Allies agreed that an interim government would be formed from both the pro- and anti-communist sides, with free elections to follow. The Soviets had other plans, though, and promptly turned the exhausted and battered Poland into a nondemocratic satellite country, which it remained until 1989.