You mean when the various attempts at a anti-nazi pact were thrown aside by the British and Poles, even when Romania was ready to have the Red Army flown over their borders to Czechoslovakia but Poland and England threw a hissy-fit and Poland signed a pact with the Nazis to get a chunk of Czechoslovakia? You mean after Britain sent diplomats on the slowest route to the USSR to discuss an alliance only for them to arrive and be told by England that they had no power to sign anything and to just waste the Soviet's time? Which pissed the delegation off and led to them admitting the truth to Voroshilov. Or England refusing talks because they included the Baltic states which they said shouldn't be involved, while also rejecting talks because they didn't involve said states. Or when Chamberlain wrote to his sister repeatedly that he was actively trying to get the Nazis to attack the USSR. Or when Poland's government responded to France by saying that they literally view the Soviet people as subhuman and that they'd rather by occupied by their fellow "pure" Germans.

You mean that context?

Poland was going to be invaded, the Soviets hastily saved a chunk of its army and people, which happened to include almost entirely territory that is not actually Polish. Or is Belarus "part of Poland" same with Vilnius?

They partitioned Poland and signed a non aggression pact with the Nazis because their repeated attempts to get Britain, France etc to align against them were rebuffed, which every anti communist lib leaves out when condemning the USSR for it