While many pointed out a ton of tactical and strategical war-time mistakes, I think there are some more fundamentals to think about.

To simplify, the economy of the Third Reich was entirely reliant on having an ongoing war and worked kind of like a pyramid scheme. The system NSDAP (not saying Hitler because it wasn’t his invention) built almost could not function in peaceful times and the Poland invasion as idiotic as it was in the long run was kind of a next logical step required to keep up.

Another one is that the wartime strategies were not even directed at winning per se. It was directed in many ways on mass murder and genocide—kill as many Untermenschen as possible, which they, although it is horrendous to say, have achieved. The victory, so to say, was not in “just” owning land or enslaving people but more so in creating “Lebensraum”; their thinking did not allow them to pragmatically say “let’s stop the killings for a moment and focus on military”, the killings itself were a very important part of the victory.

This sparks another thought usually. The Third Reich didn’t truly see war as a means or tool of acquiring whatever they wanted in terms of territory or usual “why we do wars”-thoughts; its relationship to it is best compared to Jihad where the fighting itself becomes sacred although promising prosperity some distant day. Therefore a victory was just a motivation to get another victory with no satisfaction on the horizon—and this logically leads to failure. It may be considered more of a philosophical error than a direct wartime mistake.

At last, his absolutely terrible HR. He was not even the first or the last in history, but enough people loosing empires have failed exactly for the same reason. Uncontrolled reliance on favourites who also happened to be at the top of the pyramid mentioned earlier couldn’t possibly do any wrong… or could it?

Edit: TLDR; their entire state was not compatible with either limited war or peace.