I agree with -dead-. If there ever a book-length topic, this is it. Let's start with a small dose of the politics and do the economics later.
The US sent carriers during China's sabre rattling in the 90's, but the mutual security treaty of 1954 was with Formosa. Taiwan is now a non-entity without diplomatic (or legal) recognition so the treaty would be considered no longer valid by the US if desired.
One must keep in mind that the change in China/Taiwan status during the Nixon years was based on the (astounding) premise that the US relationship with China would help the US keep Japan under control. The official US policy in the 1970's was that Japan must still be contained.
Chinese leaders know well that taking something is easy if you persevere. China has built outposts on small islands controlled by Indonesia and Thailand and along major shipping routes without anyone standing up to them. They just did it.
The colonization of Formosa by Japan is considered by Chinese to be the greatest humiliation in the long history of China. The fact that Taiwan is still not part of the fold keeps this wound alive, and Chinese politicians are adept as any western politicans in funneling their society's opinions for their own purposes.
Pride is just as strong a motivator of Chinese government actions and society as any other nation. Maybe more so. China's leaders know quite well that any action against Taiwan would be justifiable and supported in the eyes of the people who have been 'prepped' before the operation. The US and UK proved how easy it is to prep the population of a democracy with flag waving, even in societies with a free press.
I believe China learned some other tremendous lessons from the invasion of Iraq that make a big difference in today's dynamic. The lessons being that there is no such thing as international law; the US is not intersted in using the position of sole superpower to advance any noble casuses such as global stablity, it is interested in advancing its own interests only; the US 'one China' policy in essense endorses any action to bring Taiwan back under China's control; the use of military force to 'return' Taiwan will not be challenged by the UN.
Will the Chinese invade? If necessary, they will. They simply made it 'lawful' now. They are sitting back gauging the reaction to their move to make it lawful.
Do they want to invade? No. they would prefer not to. They are just keeping their options open because conditions change. Hong Kong still isn't as stable as they would like and they are learning now what works and doesn't work in reunification.
Will the US defend Taiwan if there ever were an attack? I would say the US is more interested in pushing Taiwan toward accepting peaceful reunification, then supply or defending Taiwan. The US would rather save face by pushing Taiwan toward voluntary reunification than trying to push China right now. What choice would Taiwan have when the US says, "Reunify or fight China on you're own."
The economics of this Japan/Taiwan/China/US puzzle are pretty complicated, so we'll leave that for later.
