Adolf Hitler wanted to attack in the west immediately after the fall of Poland, but unfavorable weather kept postponing the offensive. Then a copy of the plan fell into Allied hands.
With the other Great Powers involved in their own wars, Stalin and the USSR are now free to claim the territories Germany granted to their "sphere of influence."
When Germany invaded Poland in 1939, it freed the Nazis from having to worry about international opinion. Hitler and his followers could now do as they pleased.