But it is also a big game of chicken. If the mainland decided to invade (and they are not in a position to do so) the US would not start WWIII by going into a direct war against them.
First: US make a lot of promises to a lot of countries. If they are not willing to go to war over Taiwan, that makes most of those promises (like NATO pact) basically worthless. So US has to defend Taiwan.
Second: Military confrontation between US and Chinese forces does not mean WWIII - they won't start shooting nukes, it will be more conventional war. We had such examples during previous Cold War: Korea, Vietnam, Afghanistan.
They are not willing to do to war over Taiwan. The opposite would be insane. Equally China is not willing (and able) either.
This is a balance of threats with both sides knowing where to stop.
The US are obviously not stupid and they never committed to intervene against China. They have just committed to 'help' and make sure that Taiwan can defend itself, which is very different.
> Military confrontation between US and Chinese forces does not mean WWIII
It does when the US directly attack the Chinese over Chinese territory.
I.e. the US would be declaring war on China. I would not let anyone who does not see the insanity of that prospect have a job in government.