Dont forget the two countries signed Munich agreement with Adolf Hitler in 1938.
They also signed an alliance with Poland, and declared war on Germany when it invaded.Both Germany and the USSR wanted Poland . . . and much more. Including each others territory. After the German blitz of France, of course, Stalin knew a lot better than to pick a fight with the German army. Having purged his officers, Stalin had good reason to know that his army was not well led.
Only the German invasion of the USSR made any agreement between Stalin and Churchill possible.
“They also signed an alliance with Poland, and declared war on Germany when it invaded.”
That was a pathetic joke.
Britain and France declared the war on Germany, but were reluctant to fight for Poland. The only option was an offensive launched by allies that would force Germany to withdraw troops from Poland territory.
It didn’t happen. After a brief advance, French troops backed down to avoid the battle.
As Germans said:
Poland paid price for Warsaw government’s illusions about its allies.