And by China's definition, that's just a civil war if it happens

And by the definition of one of the two major Taiwanese political parties as well. From Wikipedia:

"The Kuomintang holds the "One China Principle" and maintains its claim that under the ROC Constitution (passed by the Kuomintang government in 1947 in Nanjing) the ROC has sovereignty over most of China (including by their interpretation both mainland China and Taiwan)...Former ROC President Ma Ying-jeou had re-asserted claims on mainland China as late as October 8, 2008."

"claims on mainland in 2008"? Do you have a source or link for that? It's not here: https://en.wikipedia.org/wiki/Ma_Ying-jeou#View_on_independe...

"Ma said under the ROC Constitution, the ROC “definitely is an independent sovereign state, and mainland [sic] China is also part of the territory of the ROC.”

The interview was published yesterday.

Ma said despite the stipulation of the ROC Constitution, Taiwan cannot recognize the existence of another country, nor does China want to recognize Taiwan. In other words, under the ROC Constitution, “mainland China” is not a country."

from http://www.taipeitimes.com/News/taiwan/archives/2008/10/08/2...

via https://en.wikipedia.org/wiki/One-China_policy

Ma Ying Jeou and the KMT have some persuasive arguments; there was an interview on YouTube where he eloquently explained why these claims exist; the mainland Chinese Communist party came to power without being elected by the people. Also, when Japan signed its surrender papers giving up its claims to annexed territories, the sovereignty reverted to the KMT government.

I mean, the KMT of that era would have had even less of a claim of being the legitimate representatives of the people for any reasonable definition of "the people".