Eventually a secret agreement with Nazi Germany allowed Germany and the Soviet Union to successfully invade and destroy the Second Republic in 1939 . The following years of Soviet repressions of Polish citizens , especially the brutal mass murder in 1940 , known as the Katyn massacre , of more than 20,000 Polish officers and its subsequent Soviet denial for 50 years , became additional events with lasting repercussions on the Polish – Russian relations . In 1944 the Polish Home Army timed their capital 's uprising to coincide with the approaching Red Army on the eastern suburbs of the city and the retreat of German forces . However , the Red Army stopped at the city limits and remained inactive there for several weeks . Also , the Soviet Union did not allow its Western Allies to use its nearby airports for airdrops into Warsaw for several weeks . This allowed the German forces to regroup and demolish the city while defeating the Polish resistance and causing between 150,000 and 200,000 civilian deaths . The tragic circumstances under which Poland 's capital was liberated further strained the Polish – Russian relations . At the Yalta Conference in February 1945 , Stalin was able to present his western allies , Franklin Roosevelt and Winston Churchill , with a '' fait accompli '' in Poland . His armed forces were in occupation of the country , and his agents , the communists , were in control of its administration . The Soviet Union was in the process of incorporating the lands in eastern Poland , including the mass expulsion of the Polish population , which it had occupied between 1939 and 1941 , after participating in the invasion and partition of Poland with Nazi Germany . Stalin was determined that Poland 's new government would become his tool towards making Poland a Soviet puppet state controlled by the communists . He had severed relations with the Polish government-in-exile in London in 1943 , but to appease Roosevelt and Churchill he agreed at Yalta that a coalition government would be formed . The Soviet Union supported Polish demands to be compensated by the loss of the former East of Poland , from which 2-3 millions Polish citizens were expelled , by German lands east of the rivers Oder and Lusatian Neisse which had homed 9 million Germans . Stalin allowed Polish authorities to man the Oder-Neisse Line as border , notwithstanding the lack of international consent for the new border , to prevent Germans from returning to their homes after the German capitulation
Katyn massacre
Katyn massacre
Poland–Russia relations