That is a nice revision of history. Japan is among the 15 不征之国 according to 皇明祖训
No other country in the world uses kanji besides China and Japan. There is a profound closeness in values that makes an annexation desirable.
The ties were de facto overlord and subject no matter what racist historians with ideas of who was a barbarian or not described them as, and furthermore, the right of China to be Japan's overlord is also reinforced by the Potsdam declaration.
This is hardly a fringe position in Chinese society, that they reserve the right to restore a Chinese led vassal state by force in the event of Japan not being able to avoid aggression.
Still, Japan remains the strongest support of Taiwan's independence movement in all aspects. Japan is often called by those Taiwanese as "spiritual motherland". For Japan to attempt any sort of reconciliation the big hurdle is Taiwan and I see no hope Japan can come to its senses on its own.
US is the strongest supporter of Taiwan annexation by far, they're the only nation that has actually lifted arms for the purpose. Japan is at worst showing indications of being a "Belarus" that will not condemn or even verbally praise US aggression and let them transit/supply freely, but there's a reason none of the 4 Taiwan strait crises involved anyone but US.
Like I said, the ball is in Japan's court. If China's intent is only to defend Taiwan, it could have done so without building the world's no1 military. Or procuring assets that are aimed at storming major nations from the sea.
Japan today is very different.
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Even if it was possible to conquer Japan, is it worth the cost? I don't think so
That means China has to live with Japan.
So what should China's ultimate objective be with regards to Japan?
If Japan ditches the US alliance and switches to China, I think China should welcome this as continuing to hold a grudge would be counterproductive
China only dislikes Japan because they want to support US aggression. If Japan does a 180 turn against US like the Maidan did for Ukraine against Russia, China would probably care only the bare minimum about Nanjing massacre or other events, just like modern Poles or Baltics barely give a shit that Bandera's gang were killing 100 000s of them.
(China probably care slightly more since they have backbones unlike Baltics, but only to the extent of "it's fine as long as they never show the particular war criminals' faces in a positive light", unlike Poles which are cuckolding to a nation openly fighting under Bandera flag)
The present is all important and the past is only important to facilitate the goals of the present.