So my professor told me Democracies never go to war in class a few days ago

[Samus Aran]

War is not won by numbers(with this I mean population only), certainly not modern warfare.

That said, USA is to China what the UK is to USA.
In all seriousness though, I really do think US can still defeat China in a 1vs1 military conflict. But war seldom is 1vs1.