USA vs China , if china will declare war against USA, then Chinese have to put their army on the boats,jets etc and have to cross Pacific Ocean witch USA wont let them do it, they will just bomb the shit out of them before Chinese will reach shore of the USA.
Of course Chinese army is huge and have big advantage in men power, while USA army have advantage in technology, and technology is big advantage so USA will win the war.
And if it will come too close then USA will probably use nuclear weapon.