Was Hitler's Single Biggest Mistake.......

Giving up on the painting.

Yes, I would say invading the Soviet Union was his biggest mistake. A close second is not invading Britain after he spent all that time bombing it.

Although one could say his mistake was in starting the war too early. If he had waiting 2-3 years he would have had more submarines, more landing craft, etc to do his work with. Germany had fine weapons, just not enough of them for the long haul.

They never had the slightest chance to be able to invade England and they never would have got it. More Submarines would not have helped, especially if the UK could get US aid as they did in real life, and having real landing craft would have done nothing to help gain air or naval superiority, which Germany could not do. The best Germany could have got would have been air parity, which is not good enough. The RN would have continued to maintain naval superiority bordering on naval supremacy.

Tne reason Germany didn't have enough weapons or other resources to win is because the British Empire even without the addition of the Soviet Union and the United States had economic and industrial superiority.

His biggest error was committing to a foreign policy based on expansion and aggression. This would inevitably lead Germany into war with all the major powers in Europe and with the United States. His next biggest error, which was masked by the war, was an economic policy that would have resulted in a economic collapse without looting Germany's neighbours.

Specifically wit regards to the Soviet Union, this was a serious error in hindsight, but conquest of the USSR and Poland were key elements of Nazi ideology. Once Poland had been conquered, war with the USSR was inevitable. The only question was which power would stab the other in the back first.

Micromanaging the war (Stalingrad is the best example of his foolhardiness) and not waiting until Italy was stronger.

We try but we didn't have long
We try but we don't belong...

-Hot Chip (Boy from School)

The only difference Hitler's management of the war made is that ha better supreme commander might have extended the war a few more weeks. A really good one might have extended it lomg enough for a couple of German cities to enjoy instance sunshine rather than two Japanese ones. The cause was hopeless when the Soviet Union failed to collapse in 1941 - something it was not going to do.

As for waiting on Italy - a year or two later and the Allies would likely have been strong enough in France that quick victory would have been impossible. Cue a war of attrition against more resilient opponents that the Axis could not win.

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.

Probably. Though nominally declaring war the US would be bigger (picking a fight with a massive economy whilst bogged down in russia isn't great) but then the US would likely of found reason to declare war on Germany anyway.