Actually, the war could have gone very badly for us very quickly in Europe. Being a history major helps a lot! Reasons the war could've very easily went the other way:
In the beginning, Germany was the technologically superior force. Had the battle of Britan lasted a week longer, Churchill wouldn't have had any planes left to fly, forcing his hand, and shortly thereafter would have had to surrender, as what the subs weren't sinking, the planes would have shifted to mopping up, starving the brits. Had Hitler held up his end of the bargin, Stalin would have been on his side a bit longer, and there would have been no eastern front to fight. Following the fall of Britan, he could have concentrated on producing weapons that were more reliable, and longer ranged, not to mention, by '48 at the latest he would have had atomic weapons, and had subs that were not able to be found by Allied ASDIC, and could launch V2 rockets off of these subs into places like New York, Norfolk, etc.
If Japan hadn't attacked the U.S. and instead had a better build up period, attacking China inward instead of spreading outward like they did, this would have put them in a good place for resources and a joint attack on Russia with Germany. This would have a two fold problem for Russia. Attack from 2 angles, and atomic weapons. Stalin was a ruthless man, but he would have been no match for atomic weapons.
The major problem with the Axis in WW2 was quiet simple. Too much too soon spreading themselves too thin. America probably wouldn't have entered the war at all had PH not have happened, and if they had, it would have been later, after the fall of Britan and China. We as a country were not in the mindset that we are in today, in that back then if it ain't broke, don't fix it. We also were not building up our forces until really '43, 2 years after the start of the war, and that would have happened whenever we actually went to war, as it took time to build up capacity.
We knew of a few of Germanies secret weapons programs, but the type 21 and 23 were something we didn't know anything about until the war ended, but we were honestly ignoring the war until PH, and had that not happened, we would probably be Sprechen sie Deutche!
I wish that I was better at modding games, it would be great to model an alternate war to the way things could have went.