During 1944 the Allied leaders , particularly Winston Churchill , tried to bring about a resumption talks between Mikołajczyk and Stalin , but these efforts broke down over several issues . One was the Katyń massacre . Another was Poland 's postwar borders . Stalin insisted that the eastern territories should remain in Soviet hands . Mikołajczyk also opposed Stalin 's plan to set up a Communist government in postwar Poland
Katyń massacre
Katyn massacre
Stanisław Mikołajczyk