After repeated appeasement efforts that the third reich used to take Saar, Austria, Sudetenland and the rest of Czechoslovakia Hitler and his generals were apparently convinced that France and Great Britain would do nothing even if the Nazi armies would roll over to take Poland despite clear messaging that this would not be the case as well as Roosevelt seeking assurances that Hitler would not invade other countries in the future. They did see a war against Britain as inevitable at this point but thought it could erupt only around 42-43.

In reality the western powers had finally had enough and the invasion of Poland resulted in France and Great Britain declaring war. They were unable to help Poland and what became known as the "phony war" would still last for many months with little direct engagements between the two sides.

Historigraph on YT does make a good case for the Phony war being somewhat of a misnomer because while much wasn't happening on land in central europe there was still war at sea and in other places like Norway/Finland.