Although the Polish government-in-exile was recognised by the Allies at that time , the Soviet Union broke off all diplomatic relations with it in April 1943 after Polish government demanded the investigation of the Katyn massacre . On April 20 , 1944 , in Moscow , the Soviet sponsored Polish Communist cell founded the Polish Committee of National Liberation ( PKWN ) on Stalin 's initiative . Just one week later the representatives of the PKWN and the Soviet Union signed a treaty regulating the new Polish-Soviet border . A year later , before the Potsdam Conference , the western Allies followed Stalin , recognized the Soviet-sponsored government , which accepted the shift of the borders westwards , and withdrew their recognition for the Polish government-in-exile . Poles were classified as sub-humans ( untermenschen ) by the Nazis , with their ultimate fate being slavery and extermination , while Germans occupied position of privileged " ubermenschen " that were to rule over Poles and other nations ; when Stanisław Mikołajczyk joined the " Government of National Unity " as a deputy prime minister in 1945 , he justified the expulsions of Germans by national terms following communist Władysław Gomułka , but also as a revolutionary act , freeing the Poles of exploitation by a German middle and upper class
Potsdam Conference
Potsdam Conference
Flight and expulsion of Germans from Poland during and after World War II