Well, Hitler was a narcissist populist politician, not a great military strategistin the first place. His biggest mistake was that he started to believe his own propaganda. He should have listened to Prussian generals instead.
Who probably would have talked him out of expanding the war after their conquered Poland.
If fighting is sure to result in victory, then you must fight, even though the ruler forbid it; if fighting will not result in victory, then you must not fight even at the ruler’s bidding. — Art Of War X:23The generals could not have feared the Führer to the point of not heeding that sound military advice, could they have?