There is no “reason” or justification. The decision to invade has already been made, all these explanations exist for the sole purpose of making it easier to do nothing about it when the invasion happens. Chinese people feel they way they are taught to feel.

China's fundamental argument distilled to its most pure form is "we are strong enough to do this, what are you going to do about it?" A world where the strong do whatever they want because no one can do anything about it is not a world anyone should want to live in.

All of the Chinese expressing these opinions from a position of safety can’t seem to put themselves in the shoes of Taiwanese. The inhumanity of an invasion is being hidden by high level ideas like history.

The American civil war comparison fails. No American sits and thinks about invading the UK to “complete” our revolution and that’s much closer than your scenario.