One dark reality is Stalin's anti-Semantic overture to Hitler: he appointed Vyacheslav Molotov in May 1939, shortly before the pact, as a replacement for Maxim Litvinov, who was of Polish-Jewish background.
It is also curious that the territory Stalin took under the agreement amounts essentially to what tsarist Russia acquired under the Partitions of Poland (1772-95). And most of that territory remained part of the Soviet Union after the war. In fact, Stalin took yet more land from Poland in the conflict’s aftermath, “compensating” with formerly German-majority areas such as East Prussia.
The accompanying ethnic cleansing included de-Germanizing land given to Poland, and moving Poles from western portions of today’s Ukraine and Belarus, and placing them in the formerly German areas of western Poland. Naturally, these policies exacerbated an already-massive post-war refugee crisis.
I became somewhat more acutely aware of these issues two years ago when translating a letter from Rusyn. The author had lived in Transcarpathian Ruthenia, which had belonged to Czechoslovakia during the interwar period and was then assigned to the Ukrainian SSR. But she also had relatives in the southeast corner of Poland, and there were things she dealt with only obliquely in the letter - writing, as she was, from her new home in the U.S. to yet other family in the Soviet Union - for fear of reprisals against the recipients of the correspondence.
Still, the Polish population of the Grodno region, for example, one of the most heavily Polish areas attached to Belarus, has managed to survive. It has even thrived in the years following the dissolution of the USSR and now stands at around 230,000.