The concept of a USA versus the rest of the world conflict simply doesn’t work as a reasonable hypothesis.
Were the USA a land mass surrounded by many differing nations like Russia - the possible result of such a conflict could be argued.
But the US navy rules the seas and it would be virtually impossible for the rest of the world’s armies to actually reach the borders of the US (or reinforce Mexico or Canada) without huge and unacceptable losses.
Incidentally, with such overwhelming superiority, it’s very regrettable that the US never used that power as judiciously as did the Royal Navy, that with similar dominance, kept the world at peace for a century.
Viewing the performance of the US army (and Marines) in recent years, I don’t think they’d last long against any one of many different combinations one could craft from the armed forces of many differing nations.
Even in the apogee of American fighting prowess in the second world war - only a few specific units actually measured up to the best of both the Axis and allied armies.
These were units like the 82nd and 101st airborne and some marine divisions fighting the Japanese in the in the Pacific.
But even these would have stood little chance against, say, the Waffen SS - but they were devastated, totally outnumbered, fighting vast tank armies on the Russian Front and only a decimated fraction ever fought in the West.
Recent wars show that the Americans have more to fear from so-called ‘friendly fire’ from their own forces, rather than the enemy!
Slight exaggeration, but unreserved reliance on massive firepower brings its own dangers.