Hitler saw himself as a natural born strategist. Which he was not. He got really, relly lucky in the beginning. Mostly because his opponents did not understand 'blitzkrieg' and they where taken aback.
Then, during the campaign against Stalin, Hitler kept on dreaming, while his opponent got a bit smarter. And Hitler totally underestimated the reserves that Stalin could drum up. In the war against the Soviet Union German soldiers could kill 10 to 20 Russians to the reatio on 1 German. But what did that matter since the Russians where 250 soldiers against 1 German? (At least) The Russians just kept comming, and comming. No end to the nuber of troops, no regard for any Russian soldiers life at all!
Hitler miscalculated this ratio. He did not listen to his generals when they tried to put things into perspective. If someone tried, he would get brushed aside or loose his job. So many where afraid to 'talk some sense' with him.
That became Hitlers undoing eventually. Even at the last few days, in his bunker, Hitler thought that the 9:th army should come to rescue everything. He told himself that general Wenck would soon be there. When, in fact, the 9:th army was completely surrounded and Wenck did not have any troops to do war with at all.