Beginning in late 1939 , hundreds of thousands of Poles from the Soviet-occupied areas were deported and taken east . Of the upper-ranking military personnel and others deemed uncooperative or potentially harmful by the Soviets , about 22,000 were secretly executed by them at the Katyn massacre . In April 1943 , the Soviet Union broke off deteriorating relations with the Polish government-in-exile after the German military announced the discovery of mass graves containing murdered Polish army officers . The Soviets claimed that the Poles committed a hostile act by requesting that the Red Cross investigate these reports
Katyn massacre
Katyn massacre
History of Poland