There were multiple errors on all sides.
We won the war, in large part, by not making as many errors as the axis powers.
The Germans were the best soldiers in the war.
Hitler's ambition overreached his ability to project power.
I think there were multiple points where Germany could have won... but there was no clear end-point for Hitler to stop, until he was stopped.
Germany did not have the manpower or industrial might to conquer the world.
Maybe, if they had gotten the A-bomb, they might have. But they erred in their theory and allocation of resources in that regard.
I'm not sure this is true; the United States made multiple errors during the war. Our unmatched wealth, industrial might, and distance from combat allowed us to make mistakes that would have sunk the smaller countries of the Axis in a moment.
During the U-Boat campaign, there was actually information being passed from American insurance companies to subsidiary firms in Switzerland regarding departure dates of ships, destinations, and cargo manifests; this was for re-insurance purposes (to spread the risk in the event of loss). These Swiss insurance companies had business connections with companies in Germany; there was literally a direct line of information regarding ship departures and destinations going straight to the enemy, except the enemy didn't have the resources to put this information to use.
This went on until sometime in 1943 before Congress shut the operation down.
Had Germany or another smaller country demonstrated this kind of carelessness or incompetence, the effect on their war effort would have been immediate. However, because it was the country that had the wealth, industrial might, manpower, and geographical security that the United States had, the result of such carelessness/incompetence was negligible.
The graphite they used to moderate the reactions for the test data for their calculations contained just a tiny whisper of boron. Boron absorbs neutrons, and that reduced the reaction rates just enough to convince the Germans that an atomic bomb would be too heavy to deliver by any practical means.
Most certainly in 1940; and just as certainly, not in 1944.
They could not fight on two fronts, overwhelmed by sheer numbers in The East, and us and the Brits in Africa, Italy, and finally France.
They also had no amphibious capability. They took Crete with a massive airborne effort.