Russia Ukraine war, page-234156

You're still oversimplifying the situation and not recognising the complicated dynamics in play at the time including Stalin's conditions in any agreement with the UK/France along with the Polish/Soviet relationship and degrees of trust at the time and the Soviet/Polish War the previous decade.
So offering another oversimplified analysis....instead Stalin, did a deal with the Devil he was so worried about helping facilitate Hitler's invasion of Poland sparking WW2 then also invaded them and annexed eastern Poland as part of that deal with the Devil. That's what I call doing a 180 degree turn in a matter of days.
Based on the events and timing it looks like it was Stalin who was the expert at smoke and mirrors games.