Let us not forget the anniversary of Soviet attack on Poland in 1939.
Russia did not even declare war on Poland. Due to this fact when Soviet armies entered the East there was much confusion. Some expected the Soviets to fight the Germans, some suspected they were about to annex Polish eastern lands. Our high command was equally confused and issued orders that were unclear to most commanders. That's why most Polish forces in the East simply surrendered instead of fighting against invading Soviet units.
And then the Soviets murdered 20 000 of our officers. Without a reason or trial.