Was Hitler's Single Biggest Mistake.......
......to invade the Soviet Union, or was there another that was even 'bigger' ???
SAVE FERRIS
Not owning the atomic bomb first. Everybody got guns and tanks, he didn't have a super weapon to match his big talk. Had nothing in his back pocket. Even if taking England or USSR; you still have to face the Atomic bomb among Uncle Sam. They had faith, but not the cold war.
Which leads to the actual biggest mistake: persecution of the Jews. All you have to do is look at how much of the Manhattan Project brain trust was here because of it.