I have a friend from Poland whose family is from the eastern half of the country. Due to the Hitler/Stalin Peace Pact of 1939, the country was divided into two with the Bolsheviks taking control of the eastern half. The Communists butchered most of her family; she said as horrific as the German occupation of western Poland was, it was also extremely bad in eastern Poland under the Bolshevik Soviets. Due to the necessity of war (Stalin’s Russia became an ally in December 1941), the Bolshevik atrocities were never publicized, none of those who led the effort were ever prosecuted and “Uncle Joe” was our friend.
There were also a lot of Ukrainian atrocities against Poles, as documented in the movie “Wołyn”.