Let's rein in the pointless and OT popularity contest and ego trips please. Those things have nothing to do with what this thread is about.
Japan going aggressive and looking at pre-emptive attacks is just plain silly and ultimately self defeating.
Neither the Russians or the Chinese are overly concerned by Japanese military might when both benchmark themselves against the US military anyways.
Neither China or Russia would doubt that America would honour its treaty obligations and come to Japan's aim if Japan was attacked. But if Japan initiated hostilities by shooting first or launching an invasion of Russian held islands, well that's an entirely different matter altogether.
Legally and politically, America is not obliged to help Japan on a war of aggression, and against the likes of China or Russia, a direct military conflict will almost certainly result in US casualties levels not seen since Korea or WWII, and that's a best case scenario.
If Japan really was stupid enough to attack China or Russia, I think there is every chance America will hang them out to dry. And it would actually be in their best interest to do so.
Staying out of the conflict means no America casualties and no attacks on American bases and assets in Japan. It would also serve as a much needed reminder to the Japanese of who actually calls the shots in that relationship, because if the Japanese went against America's wishes and interests and started a shooting war all by themselves, that will piss off the Americans almost as much as whoever the Japanese attacked. Can't have the tail wagging the dog now can we?
There will probably be some unspoken ground rules, and so long as China or Russia don't go too far and back off when America asks, I think America would be happy to let the Chinese and/or Russians teach Japan a lesson and push Japan even more firmly under American control.