Just a little something to add to this thread:
17 September 1939: The Warld War II began with Nazi and Soviet invasion of Poland in September, 1939.
www.17september1939.com
In general, both the German people and the Russian people have never wanted to obliterate the Polish nation. However, certain German and Russian regimes have done so. Both knew that you didn't need to kill everybody, just those who resisted and make the rest into obedient serfs/slaves without their own language, culture and leadership. This is effective obliteration of the nation.
Before 1939, those Poles who had been left behind the Soviet border as established by the Treaty of Riga in 1921 were targeted by an NKVD "National Operation" personally authorised by Stalin.
See:
Stalin apologetics - RationalWiki
In 1939 both the Nazi Germans and the Soviet Russians were able to begin to carry out their plans to obliterate the Polish nation.
If the war had then gone the way the Stalin expected it to do so: German attack on the West, stalemate in the trenches for several years, exhaustion of the combatants, then the Red Army steamrollers through aided by Communist fifth columns and partisans across western Europe. Poland and the Polish people would exist in name only as an SSR with Russian as the official language, a re-written history and an enforced Russified culture. Eventually to become oblasts of either the RSFSR or the Ukrainian and Belorussian SSRs after several vainglorious uprisings snuffed out the final sparks of resistance, just like the nominally autonomous Congress Kingdom of Poland was snuffed out by the Tsars in the 19th century, becoming directly ruled Gubernatoria of Russia.
Because the war did not go that way and his erstwhile ally of convenience attacked him first. Stalin was unable to fulfil his intended plan and due to the change of circumstances vis-a-vis the West was forced to pretend to accomodate the Poles. This was necessary in order to keep the "fellow travellers" and the "useful idiots" of the West on side so that the plan might be fulfilled in future.
The Poles who did suffer first the Russian occupation, then the German occupation, then the renewed Russian occupation and ethnic cleansing of annexed territories see no difference in the attitudes of the Nazi German and Soviet Russian regimes. My maternal grandparents remembered the oppression and the attempts at Russification in Tsarist times. The Soviets were horribly worse and there was no difference in their treatment of Poles. Incredibly, in contrast to Russian officers a few German officers were actually remarked as being somewhat civilised and attempting to distance themselves from Nazi excesses.
My maternal grandfather was separated from his wife and children for 18 years when he was deported by cattle truck to a Siberian Camp in Feb 1940. He estimated that a quarter to a third of those on board during the month long journey died - mostly the sick, the old, women and children (his wife and kids had avoided this fate by visiting relatives in a neighbouring village the night when the NKVD descended on their town). They were being worked to death logging the forests on meagre rations in all weathers. He would have been left there to his fate, had it not been for the pressure of the Polish Government in Exile and that of the UK on Stalin to release the surviving Poles after Barbarossa.
Under General Anders (released from the Lubyanka and who had been spared the fate of his brother officers at Katyn due to the Russians believing that they could turn him into a quisling puppet) the surviving Poles including many women and children) were determined to get out of the Soviet hell-hole and made thier way to Persia (with no pro-active assistance from the Soviet regime) later forming the Polish II Corps of the British Eighth Army.