But even if you were contemplating a direct war the issue would be that direct wars against Russia or China are not realistic because they are nuclear powers and, in any case, too big to invade. Any direct war would certainly require a major aggression from these countries against the US themselves, which they obviously won't do in the same way as the US won't directly attack them.
It's all boils down to relative strengths and outcome of a war. There are wars that are never worth starting.
The question is, do we want to end this idea of "taking back" things that were "ours" in the past? What even is "ours" and when do you get to call the cutoff time? What is it, 1800 AD, 1200 AD, or 2000 BC?
China itself was made up of smaller civilizations many ages ago... so if I'm a present day decendant of one of them, do I get to claim that part of China as my own?
Also, when did the CCP ever control Taiwan? Never. So how can they take it back?
Again, this has been played to annoy the PRC, standard divide-and-conquer tactic, but the reality remains that no country is going to go to war against China over this, or in general.
It's ridiculous. The whole point is we don't want to have a strength-based order in the world. If so, it will become winner take all and lots of people will die.
If China wants to "finish" a civil war and start the attack then fine, they have the blood of innocents on their hands.
I don't think the PRC is in a position to finish off the ROC, and I don't think it would be in their interests, either.
But equally I don't think that the US want to intervene directly should the PRC take over Taiwan. This would be madness. The problem is thus to imagine how the US would manage to politically 'sell' their inaction. But at the same time, would the PRC dare to take the bet that the US wouldn't intervene?
Taiwan is not much different from Crimea on many aspects, but it is different in one key aspect: It's been given much more international prominence, mostly because it is a pawn to annoy the PRC. But the downside is that it makes non-intervention (which is the reasonable course of action, like in Crimea) more difficult to sell.
Then, should the PRC invade (which is unlikely) while I'm sure that the Taiwanese would put up a good fight they would lose anyway. There would be plenty of grand speeches and maybe sanctions, but the reality is that everyone would just be looking for the right time to recognise the situation and move on (that's what happened with the existence of the PRC itself, and I'm sure is what will happen with Crimea).