If I "knew" that eventually Germany would invade the US, I would have to un-know a lot of things.
If you travel from London to Istanbul, how many different nations, cultures, languages, and religions are you going to encounter, just going in a direct line?
There is less physical distance between those two cities than there is between New York and Los Angeles.
In the time we're talking about, almost every single state in between NY and LA had foundries, machinists, farms, food processing plants, ore, fuels, wood, guns, guns, guns, guns, guns, and free people who knew what to do with them.
That we tapped into all that so effectively is the only reason why we were able to do what we did in Europe.
If tiny little Germany wanted to invade the US in the 1940's, and I was president of the USA, all I'd say is "bring it, chumps".
Today we have remote-controlled flying killbots, and we can't subjugate some stone-age hillbillies living in caves in the world's armpit.
Germany's defeat by us was a foregone conclusion when we started. To suggest they would be able to do the reverse is ridiculous.