If we have a Sanders, or Hillary then it is significantlyless likely that the US with intervene, especially if the war is triggered by a dispute over the Diaoyu Islands, which is not explicitly recognized by the US as being sovereign territory of Japan. This little loophole will allow the US to justify non-intervention.
Furthermore, if it looks like the Japanese can emerge victorious even without American aid, then the US is even less likely to intervene. China will be "checked" while the US-China relationship will remain intact.
But with a Republican in the White House and a losing Japan, then US intervention is almost guaranteed.