The German attack on the Soviet Union in June 1941 changed the course of World War II and with it the nature of Polish-Soviet governmental relations . Pressured by the British government , the London-based Polish government-in-exile , led by Władysław Sikorski , signed an agreement with the Soviet Union , which included a Soviet recognition of the Polish government . A Polish army was formed in the Soviet Union , but was soon taken out of there and into the Middle East by Władysław Anders . The Katyn massacre perpetrated by the Soviets on Polish POWs was revealed and the Soviet Union " suspended " diplomatic relations with the Polish government . Prime Minister Sikorski was killed in an airplane crash in July 1943 . These and other factors , including disagreements about future borders , caused the Polish-Soviet relations to deteriorate
killed in an airplane crash
1943 Gibraltar B-24 crash
Polish Workers' Party