Hitler made the mistake by attacking russia, I agree that he needed to have a good amount of men on that side to hold of the russians if they did try to attack, however he should have killed off britain and used it's resources. Then devoted a good bit of supply and other forces (Rommel) to attack Russia instead of attacking preimtivly to soon.
Now originally he did great in the opening fighting but he went to far in to soon, after crushing much the opposing forces he should have stopped and waited for supply lines tobe more secure and wait for more troops, not tons but a good amount of reserves to stop an encirclement like the one that destroyed the 6th....Obvious blunder, never leave your flanks weak, the reserves would have been able to help the parts getting attacked and would have probably stopped the encirclement.
On Topic, Hilter may have pulled most of his forces into the Hilly part of Italy (The Alps) which would have been a great defensive spot, keeping the medertrain and the whole Italian pensula would have gave him a nice little defense and opened him up to asia and a possible tie with Japan. By taking africa we pressured him to send troops to aid Italy, thus weakening him and giving him a bit of mesaure. Over streching him ourselves, and indeed Rommel, the Desert Fox was short on oil I believe. He was an amazing General but never given the resources he needed to really do the damage he could in africa.