Tokugawa-Koxinga Alliance

Tokugawa-Koxinga Alliance to retake Mainland China from the Manchurian Qing in the 1660s. Could a new dynasty that encompasses China & Japan, & possibly Korea emerged out of it, assuming Koxinga didn't died of malaria.
 
Timeline doesn't really work.

The 1660s saw the Tokugawa shogun underage and so a regency and extended power struggle. That's not a recipe for a decisive foreign move.

You could either butterfly that or have Japan involve itself earlier (prop up the southern ming in the 1640s) or later (sail with the zheng family to said the revolt of the three Feudatories in the 1670s) but frankly after the disaster of the Imjin War getting involved in another war in mainland asia is not going to be popular. You could butterfly the Imjin war, of course, but that's going to effect whether the Tokugawa gain power in the first place and also whether the manchus conquer the Ming.

I get the idea that japan is scared of european influence and koxinga and the zheng clan are opposed to manchu power over china so an alliance works for both. Japan get a powerful ally and Koxinga gets the aid he needs .

But it's simply not how either koxinga or the japanese reacted in our time. The japanese didn't go looking for allies, they instead isolated themselevs, stopped the red seal ships to south east asia, broke off relations with thailand, etc. And Koxinga attacked both holland and spain, he wasn't really looking for an outside backer (though obviously he had family connections to Japan which might make it different).

You're going to need some major changes before both sides decide to go for it.
 
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