That is an interesting view.
I always believed it was when Germany started fighting on various other fronts, instead of concentrating on knocking Britain out of the war.
By getting involved in North Africa and the Balkans and Greece, to help out Italy, and then launching the attack on the Soviet Union, without obtaining a definate, outright victory, I always believed Germany over stretched themselves.
Another thing I always thought was a key, was the armed forces of Germany were not designed for a long drawn out war, especially the Luftwaffe which was seen as airborne artillery for the army and possesing no long range strategic bombers at the outset of the war.
Sempre in merda profundum