It depends on who is the US President at the time and whether Japan is winning or losing on its own.

If we have a Sanders, or Hillary then it is significantly

less 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.

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