If the entire world is against the USA, then that means Mexico and Canada too. There are already "footholds" on the continent. There need not be an opposed landing or a suicidal hike across the Alaskan landscape (seriously, invading through Alaska is not an option).
And tell me, how long is Canada going to last from the declaration of hostilities? Long enough for the major powers to mount a massive reinforcement?
I'm surprised no one picked up on the thread of my "what happens to Japan?" question. It's not like it's a trivial question mind. The fate of the world's third biggest economy is kind of a big deal.
The problem with the US navy is that it'll be contending with rapidly expanding navies.
It's questions like this that make me ask "why the fuck does this war happen in the first place?" and "Precisely how long are we talking about?".
Firstly, the UN can't be involved, the US has veto power there, so some whole new organization has to be formed specifically to combat the US. And for this situation to occur, numerous countries that are either dependent on the US (Iceland, Japan, Phillipines, Israel), or highly vulnerable to the US (Canada, Mexico, Costa Rica, literally all of Central America really) have to declare war on the US, knowing precisely how shitty it is going to be because of it. How determined are these countries in the first place? There are real differences if the entire world is dedicated single-mindedly to annihilating every single last US citizen than if, say, the major foreign powers of the world founded a "Coalition of the Willing" of sorts. The latter scenario is actually possible, for one.
Beyond that, I tell you that even best case scenario for total war is "Fortress America." Even if the world resists America like it did the Third Reich (and note, the world did not resist the Third Reich as much as the world is resisting America here! in WW2 you had collaborators and such, neutral states such as Sweden and Switzerland, etc. So whatever caused this fantasy scenario is
America being more widely and deeply reviled than Nazi Germany), as long as the US can set up some sort of Vichy Canada (or an "Independent State of Quebec" ala Croatia if Vichy Canada proves unviable) situation, it can hold out for quite a long time due to American trade being primarily with its neighbors.
So then we get back to "What does the world want out of the US?" and "How much is the US/World willing to pay for that?" If the answer is "a decades-long siege" we start getting into the longer-term "Fortress America" ideas, then we enter the realm of fantasy as we discuss what military forces either side could or could not be able to produce in the near-to-distant future. Again, it would be
really interesting fantasy, a very good book setting, and if someone wrote that I'd read the hell out of it, but it's not really armchair generaling at that point.