Can the US win a War with China and Russia simultaneously?

The US would never fight a conventional war with China or Russia, but might support a country that did (as with Afghanistan from 1979 to 1989). Russia has a much larger conventional army, and China an even larger one. Any armed conflict would likely lead to a nuclear exchange, either limited or total. Russia has parity with the US in nuclear weapons, while China is still a distant 3rd.