Japan would also side with Australia in such a battle, methinks...South Korea...not so sure.
One prophetic source claimed that Japan would side with China in invading the USA and be given Hawaii as a reward. We shall see. Would be a pretty dramatic shift. Lots of hostility between Chinese and Japanese over WWII atrocities.
But, I suspect China could make Japan some offers they couldn't refuse. And, I suspect there are some Japanese Nationalists they could link with who'd like to take revenge against the USA.