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.