There is no such thing as "war" with a single opponent, not anymore. Sure if they want to pretend that a situation could arise where two countries go one on one exists, then ya, they would always win. While they are at it why not pretend that no one gets access to diplomacy, intel from other countries are off limit, and no imports during said war.

If you build a constraint where the situation to arise is unreal, then unreal outcomes are inevitable... yet they won't reflect reality, which is fine sometimes, but in the case of this specific, I'd say it's kind of meaningless if that's the interpretation you take.

It's like saying Leonidas can take on the whole persian army... so long as he is allowed to fight each member of their army individually and is given ample reste when requested. He'd still be a baddass, sure. But it kind of changes the whole dynamic of the initial question.

And ultimately when I read "if my country fought another country, who wins" and the the guy claimed that us could beat every other nation no problem, I interpreted it as "If I shot nukes at russian, how many seconds will it take for every nation with a nuclear arsenal to retaliation in kind."

So maybe I missed the point personally... I guess if they keep it strictly to guns. Only use their standing armies. No cyberwarfare, no calling allies, no appealing to the public, no propaganda, no ICBM's. Basically disregarding everything that makes modern warfare modern, then ya, USA ALL THE WAY BABBBY. but if not, then the situation is MAD and it's game over everyone, thanks for playing.