> only if you accept China's definition of what is rightfully China.
In the same way, when the bank robber says that he was just withdrawing his own money, we do not accept that the bank robber has authority to declare "what is rightfully his money".
But the practical answer is, under Westphalian order whose principles are enshrined in UN Charter (again the organizing order post war), PRC has authority to declare whatever she wants within her domestic jurisdiction, of which Tibet and Taiwan is formally recognized to be part of. UN has no position on legal/procedural merits PCA/UNCLOS ruling for SCS. That's a messy situation that gets resolved same way all territorial problems in Westphalian order gets resolved in lieu of treaty/accord that formally assigns ownership, territorial control with lots of force. So Chinese authority is derived from the same post war order as everyone else.
And private persons with human rights have the authority to name territorial conquest by its rightful name.
You might assume that I sided with China mainland. Well, that not where I stand. Neither do I side with Taiwan, nor US.
I stand with the safety and prosperity of people.
Personal attacks are against site rules, and you've been here long enough that you should know that.
> China had no ambition for territorial expansion other than defending herself. That's the pattern for 2 thousands years.
China occupied Tibet. China invaded Vietnam twice. The first time, they stayed for a thousand years. "China has no ambition for territorial expansion" only if you accept China's definition of what is rightfully China. A bunch of us don't, though. For that matter, a bunch of nations don't.
Specifically with regard to Taiwan: Yes, China has a historical claim on Taiwan. (And, yes, Taiwan has a historical claim on the mainland.) Technically, as you say it is a civil war.
Practically, it's two countries. They have two militaries, issue two different passports, have two different currencies, etc. Neither one wants to admit this, but de facto they are two separate countries. (So are North and South Korea, even though that civil war legally isn't over either.)
So, legal fictions aside, for China to conquer Taiwan would be a dramatic increase of their actual territory. (That's kind of implied in the word "conquer".)
And it's clearly China that's the aggressor here. Nobody's talking about Taiwan invading and conquering the mainland. But China talks about invading if Taiwan even dares to say that they're a separate country. They act like, if we all play pretend with them, it makes their claim more legitimate or something.
Speaking of legitimacy, let's note that Taiwan is the legitimate government of China. The mainland is held by the rebels.