Depends on if nukes are being used.>if nukes are used, USA and anything that's a threat to the USA gets obliterated>if nukes aren't used, the sheer diversity of militaries and terrains the US is put up against would mean that the US would have a hard time winning even if it buffed up its army to ten times the size
Diversity of foes and terrain is a biggie in war.
In in the war between the British imperial soldiers and the Indians, the Indians lost hard despite the odds being relatively balanced.
But when those same British soldiers went up against the Maori, they almost didn't win the war; and they definitely would have lost if it wasn't for the fact that the Maori had no kinds of supply lines.
Why? Because while the Indian rebels fought a conventional war, the Maori had a fighting style that had never been seen before, in a terrain that the soldiers had never fought in before.
Elaborate bulletproof fortifications springing up literally overnight proved nearly impossible to fight, even shelling them didn't work.
Just try and imagine yourself in the shoes of a 19th century imperial soldier: You're the most modern army against one of the most technologically stunted cultures in the world, and their weapons aren't quite up to date with yours, yet somehow your foe still stands a chance of winning.
Not to mention other occasions of vast armies being defied by a minor force: Vietnam and the 300 are a couple of other shining examples.
The point is, there's countless very exotic cultures out there who have never been faced with the threat of sudden and violent extinction, and some of the times a world superpower has attempted to suddenly wipe out a culture that is very exotic to it, it ends up in a fight that is far more difficult than it should be.