That is under a big assumption that Stalin's USSR was actually better than Hitler's Germany. Since there were lots of people who fled Soviet-occupied parts of Poland into German-occupied ones (hoping that life under Hitler will be less of a horror), this is not really clear whether the cure was not worse than the disease.
* Romanies ("gypsies")
* Slavs (Poles, Russians, Ukrainians, Czechs, modern Macedonians, Serbs, Slovaks, Bulgarians, Slovenians...)
The reason he didn't exterminate everyone at the same time is that he needed slaves and collaborators in the transitional period. People down the list could be used to exterminate those higher up (indirectly, but sometimes directly).
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.
While the Soviets did harbour imperial ambitions against neighbouring states (Poland, Finland), this:
> had Hitler not attacked first, Stalin would probably just attack Germany within a short period of time
is blatantly false, and insultingly so given the millions of Soviet citizens who paid a bloody price for Stalin's total lack of readiness at the German border or mobilization against the Nazis.
No it's not. The only “Historian” to say so was the so-called Suvorov, a transfuge of the NKVD. And his theories have been thoroughly dismantled by the whole academia.