[user104658]

The attack on Pearl Harbour was catalytic and prompted immediate action but the rising aggression and clear expansionism of Axis powers mean that the US was highly likely to become involved at some point - it just accelerated the tipping point. How the war would have played out from there is anyone's guess really. It would at the very least have been a LONGER war but Germany ultimately faces the same problem each time - holding the Western front whilst also trying to deal with Russia. D-Day wouldn't have happened without US resources but like I said... I think the US gets involved eventually... whether it's too late to pull off something like D-Day is the real question.

My instinct is that, one way or another, it was always going to be the case that Germany over-stretched their ambitions. They could have taken, and held, a huge chunk of Europe if they hadn't continued to push. Eventual peace settlement would probably have seen them keeping a lot of it.