The Katyn massacre was a series of mass executions of nearly 22,000 Polish military officers and intelligentsia prisoners of war carried out by the Soviet Union, specifically the NKVD in April and May 1940. Wikipedia
The Soviets were the boogeyman to the Germans aswel. Taking people in the dead of night, evicting farmers from there land and replacing them with communist sympathizers, stealing, all that nasty stuff.
I remember reading a diary of a German officer who discribed the Soviets during the first few years of the occupation as "rag tag", no standard dress code or discipline which is pretty funny given current events
Poland was literally shifted to the West to satisfy Stalin's fetish for buffer zones. They were betrayed by the USSR too during the Warsaw Uprising and later abandoned by the West after the War. If I am a Polish citizen, I would be digging up gramp's Mosin from the backyard now.