Are we just going to ignore the Molotov–Ribbentrop Pact, wherein Stalin let Hitler invade Poland as long as he got half of Poland, Lithuania, and Estonia?

Only after France and England refused a defensive pact against the Nazis, and had already given the Nazis a bunch of territory and industry needed.

Also, would you have preferred that the nazis got all of Poland, Estonia, and Lithuania in the initial invasion? Because that would have led to a lot more deaths in those occupied territories, and might have resulted in the nazis beating the soviet union in 1942, which would have basically meant that the Nazis would have won ww2.