its will depend on type of war. if china attack japan mainland or invade okinawa etc then US will get involved directly. if china only try to push for acquiring the uninhabited island, a limited conflict. US will probably just provide intel/logistic/slap economy stuff on china etc but wont go toward a direct war against china. same with china attack some island in SCS, not mainland. especially if phillippine/japan fire 1st shot, then US have an excuse not to directly involved in the conflict, but provide support behind.Wow so whats the point of you making treaties then?
US will say we support the MDT etc, or even put few carrier groups for show of force, but those event might just be bluffing. a direct conflict between US/china will devasted both, which neither willing to go there for few rocks.