However, as far as I am informed, both, Germany and Russia invaded Poland, both treated them badly and both were rather brutal back in the days.
Is there something important in the history of the relations with those two that I missed?
Russians were often ruled by non-Russians. For example Catherine the II who together with Germany and Austria participated in Poland partition was known up to age of 19 as: "she was born in Stettin, Pomerania, Prussia as Sophie Friederike Auguste von Anhalt-Zerbst-Dornburg".
Josef Stalin wasn't Russian either. He spoke all his life with strong accent in Russian, what later served as a matter of different parodies. Nicolas the II of Russia promised independence to Poland if they will fight well on side of Russia in WWII. If I remember there was no ruler of Russia (who would be more or less ethnical Russian) and who would invaded Poland just to conquer it. Even Bolsheviks were more obsessed with idea of World Revolution than just joining Poland to Russia. Neither ethnical Russians ever genocidized ethnical Poles for just been Poles if I know it correct. It comes to a certain contrast with following historical fact:Unconditionally, attention should be paid to the fact that there can be no "Polish masters"; where there are Polish masters, and I do not care how hard this sounds, they must be killed. (...) The Fuhrer must emphazize once again that for Poles there is only one master and he is a German, there can be no two masters beside each other and there is no consent to such, hence all representatives of the Polish intelligentsia are to be killed. (...) The General Gouvernment is a Polish reservation, a great Polish labor camp." - note of Martin Borman