Greentext

Would have? Who knows?

Could have? Yes.

America had an incredibly privileged strategic position compared to the Axis, and didn't share a land border with any of them.

The fears of an Axis invasion were simply impossible. The US Pacific fleet matched the Japanese and the Atlantic was stronger than Italy and Germany combined at the start of the war, just counting battleships and screens and ignoring that America was already moving towards a carrier based fleet unlike both of them. There is no world in which America falls to a naval invasion before it had time to mobilize.

And, unlike the Axis, America was and is the world's largest oil producer. It could afford to run its Navy day and night.

The only way the the Axis wins this hypothetical is if America was alone because it went full non-interventionist (like the Republican party wanted) and the Axis conquered the rest of the world first.

That all said, these circumstances would almost certainly lead to a stalemate rather than Axis capitulation. The Axis navies get destroyed (again), but America probably wouldn't be willing to pay the blood price to invade them, and the Manhattan Project was unlikely to succeed without the contributions of non-Americans.

From the American perspective, however, a stalemate is a victory. It's a defensive war and the goal is survival, not conquest.

Tl;Dr Stalin himself made a solo victory (survival) impossible for the USSR, the US Navy and the freaking Pacific and Atlantic Oceans made it impossible for America to lose.