I'm a socialist so I sometimes visit /r/socialism and I came across a thread of people claiming the Soviet Union did not invade Poland, primarily because there was no actual legit government in Poland at the time, and the UK/USSR would have attacked the USSR if they truly invaded. They use an author named "Grover Furr" (https://msuweb.montclair.edu/~furrg/research/mlg09/did_ussr_invade_poland.html) as a source. I'm highly skeptical of this claim so I was wondering what the history community actually has to say on this issue? Thanks.
This was in fact the argument used by the Soviets themselves, Molotov tried to hand a note to the Polish ambassador (the Polish ambassador refused) and thus sent the note to all Soviet embassies:
The Polish Government has disintegrated and no longer shows any sign of life. This means that the Polish state and its Government have, in point of fact, ceased to exist . . . Left to her own devices and bereft of leadership Poland has become a suitable field for all manner of hazards and surprises which may constitute a threat to the USSR. For these reasons the Soviet Government, which has hitherto been neutral, cannot any longer preserve a neutral attitude towards these facts. The Soviet Government also cannot view with indifference the fact that kindred Ukrainian and Belorussian people, who live on Polish territory and who are at the mercy of fate, should be left defenceless.52
Its a faulty argument, since states do no lose legitimacy nor the right to their sovereignty because they have been invaded by a hostile power. Furthermore the legitimate government of Poland was still in the country, at the small town of Kuty on the Polish-Romanian frontier.
The Soviets did this because they were uncertain about the terms of the alliance between the Polish and the French/British (the treaty was meant to aid Poland only if Poland was attacked by Germany, not the Soviets) and did not want to risk a war with France and Britain. The Soviets actually tried to convince the Polish government (the same one they claimed no longer existed) to issue a statement saying the Soviets were being welcomed in as protectors, the Poles refused to even consider this.
A British assistant to Prime Minister Chamberlain wrote on the matter:
the announcement by which the Soviet Government attempted to justify their act of unparalleled greed and immorality is without doubt the most revolting document that modern history has produced’.
As for the legitimacy of Grover Furr, he's got a PHD in comparative literature and teaches English literature at a University in NJ, he's made some questionable statements about Marxism and Soviet history such as:
“I have spent many years researching this and similar questions and I have yet to find one crime… that Stalin committed.”
Whether he's a legitimate source is up for the individual to determine, but he's not an expert on diplomacy on WWII and in this case can be safely ignored.