Few people realise that the Soviet Union took roughly one-third of Poland when it & Germany invaded Poland at the beginning of the Second World War — and it kept that territory after the war. Polish society in those conquered territories was replaced with Russian society.
For me the most fascinating is the case of Lithuania. Polish children are taught about the Commonwealth, common history, fighting together... So when they hear about anti-Polish sentiment there, it makes no sense to them. People are surprised and can't understand it. That's the fault of presenting a one-sided version of history and completely ignoring the POV of your neighbors.
* They claimed that all development in Russia was led by people of German (Aryan) origin,
* Yet they focused their efforts on exterminating the social elite first, to make the society of Untermensh (slave race) fall faster.