The most egregious case is the Katyn Massacre carried out by the Soviet Union in 1940 shortly after they occupied Poland in the fall of 1939. They denied their role in the massacre for years and sought to cast blame upon the Germans for it, but evidence recovered from the site including a journal being written in at the moment of execution clearly identified the Soviets as the culprit. Soviet guilt over Katyn hung over the Yalta Conference and other post-war planning but ultimately did not play a decisive role in decision making due to the inability of the other Allied powers to exert any real influence in Poland.

Soviet actions as they moved westward towards Germany also would fit under most definitions of war crimes as mass rape, killings, and theft were exceptionally common and increased in frequency/brutality the closer the Red Army got to Berlin. Similarly Soviet treatment of POWs violated the norms of treatment for prisoners at the time and the mass starvation and death of those captured by the Soviets would also qualify as a war crime.