The Nazis had to continuously pursue their revanchist aims and win in order to maintain popular legitimacy. The German economy was also disastrously overheated and on the precipice of catastrophic collapse, which was only prevented by going to war, getting insanely lucky against France, and then plundering continental Europe for needed raw materials and wealth. The Nazis had maneuvered into a position where they had to go to war for their regime to survive, by design.

They also, like all fascists, were ideologically incapable of objectively evaluating the perceived strength of their enemies. Hitler gambled that Britain and France would back down again, and gambled again that the Wehrmacht had the capability to defeat them quickly.