Attacking Russia was the dumbest thing Adolf ever did.
Declaring war on us is either first or a close second.
Actually, Hitler had to attack the Soviet Union. Had the Nazis not broken the non-aggression pact first, Stalin planned to. Stalin had built a crack mountain force with the specific purpose of advancing through the Carpathian mountains (in terrain where the German panzers were not that useful) and cutting off German from its only source of oil other than what it bought from the Soviet Union. Without oil Hitler's war machine would have ground to a halt.
The dumb thing Hilter did was diverting troops to help the Italians secure Greece, rather than invading earlier as originally planned, compounded by not sending winter gear to the troops in the field in a timely manner.
Can't argue with that.
Napoleon Bony Part was not very smart, either.