The cost to China and/or Russia would be annihilation, particularly
to China. Since the US lacks the sheer number of bodies to throw
into such a conflict, we would almost certainly opt for 'the
nuclear option,' executing a definitive and comprehensive 'first
strike.' Whether either nation would be able to retaliate
effectively is unclear.

But ONLY that option would plunge China into such disarray that
it would be unable to organize a conventional war.