Well, the land that is now Western Belarus and Ukraine were in 1939 part of Poland and were annexed by a military intervention coordinated with Nazi Germany by Red Army. Today, no one in Poland (except few loonies) is questioning post-WWII borders, as no one sees any real benefit in changing them (I presume most Germans think more or less the same about current western parts of Poland, which were German far longer, than they were Polish).

That doesn't mean, the parts annexed by USSR were not Polish at the time Red Army marched in. There was relatively little resistance, because Polish General Command ordered Polish troops to avoid engaging soviet units. But halting quite large forces on the eastern borders of Poland which supposed to defend the land against Soviets eventually, diluted already weak defenses against Germany and spelled doom for Second Republic.

Taking over Poland by Germany was not that certain at the moment the Russians came in. I remember reading memoirs of one of the German generals leading the invasion, in which he stated, the Germans were not anticipating so long campaign and were running short of supplies. If the second front would not be open, German offensive would probably stalled on on the second line of Polish defense, which was planned on Vistula river. That means, Poland probably would lost large part of its western territory, but wouldn't be erased from the map for almost six years (of course if the French or English moved their asses at that time and fulfilled their obligations, the Germans wouldn't advance even that far, and the whole WWII madness could be stopped in September 1939).

Sadly this is true. Lots of right wing politicians view Russians as the ultimate enemy.

Personally, I admire Russian culture, like the Russians I know personally and I'm planning to start learning Russian in September. I like their design philosophy, their space program. As most of the Polish people I'm deeply grateful for the sacrifice of many Russian soldiers, who helped Poland to get rid of the Nazis and died on the road to Berlin (just look on the soviet military cemeteries in Poland, for example in Wroclaw). But in their name, and millons others who died, I can not sit doing nothing, when someone is trying to rewrite history for the purpose of feeling better (be it Russians, Germans Or Poles). All countries have some parts of their history they are not very proud of (for example Jedwabne was not easy to accept by Polish society, as I think it is difficult to acknowledge Katyn by Russians). But when we are not learning from our mistakes, we are destined to repeat them.