Japan did get Taiwan from whoever was ruling China at that time. My point is not that Taiwan never was under Chinese rule, but a) it never was really integrate

Japan did get Taiwan from whoever was ruling China at that time. My point is not that Taiwan never was under Chinese rule, but a) it never was really integrated to China like most current chinese provinces (China boasts about a 5000 year long history, loosely controlling (5% of) an island for 200 years before losing it at the very end of the their these 5000 years is not a very strong claim), b) never controlled the entirety of the island because Chinese settlers stayed on the coast, the rest of the island was under aboriginal control (which were not a uniform block) c) never cared about it (it was under the rule of a former pirate turned governor - look up Koxinga) and never did anything with it. Chiang's army retreated to Taiwan but it's a matter of semantic. It was under Japanese occupation until the end of WW2. Chiang raised the question of Taiwan to the Americans who at that time couldn't care less about that tiny island because they had so much on their plate. The Americans were like "sure, whatever, go for it", but nothing was officially signed (iirc) and Chiang and his army invaded the island and went on to subject it to an horrible authoritarian rule for the next decades. Final words, about the R.O.C claiming China. That's a sad legacy of Chiang's lunacy. And now the CCP says that if the R.O.C dares to say that they're independant (which they are, but can't say it) they will invade. So Taiwan is stuck with this nonsense because of the threats coming from the Chinese government. Also Chiang had a chance to have a seat the UN but refused because he wanted to be the only China. The rest is history. Most of what I wrote above comes from the book "Forbidden nation" by Jonathan Manthorpe (and other books but that one covers the history of Taiwan since the very beginning.) Edit: I posted that before you edit (which I haven't read yet). Edit 2: I'm not arguing against the parallel with Ukraine which I know nothing about. Just against the analogy with a hypothetical US state leaving the union.