
The Poles took Soviet land in the early 20's and committed genocide against the Ukrainians and Belarussians in their occupied territory. The Soviets were absolutely justified to liberate the land from the Polish colonial occupiers. Also, Poland had previously signed a non aggression pact with Germany, and the Polish ruling class were big fans of Nazism.
The Russians took much more than the occupied Belarusian and Ukrainian land. They literally divided Poland in half with the Nazis with it ceasing to exist as a state. Also both sides committed horrific war crimes. The polish progromed so many Jews the USA even started to intervene diplomatically. Ukrainians deported by Stalin to Kazakhstan would start digging in the ground to try and farm and occasionally find skeletons of the polish deportees sent to the same plots of land before.