[Oberon]

The US used to see China as an ally, that's why they started the whole One China policy of theirs to begin with. In regards to the defence of Taiwan, the Taiwan Relations Act states:

"the United States will make available to Taiwan such defense articles and defense services in such quantity as may be necessary to enable Taiwan to maintain a sufficient self-defense capabilities"

The level of these services is to be decided by the President and Congress, and like NATO it could range from the entire US Navy to a single frigate. It's deliberately kept vague in order to discourage Taiwan from unilaterally declaring independence and thus provoking China into attacking them.
It's a finely tuned balancing diplomatic act which has been in place, adhered to and worked for more than thirty years. Deciding to tear this all up doesn't make sense to me, but then again there's a lot about America that's not making sense to me at the moment so I guess that's nothing new.
If America wants to go ahead and take on the Chinese, then I guess that's their perogative, they have the advantage at the moment and it would be a nice big victory for Trump if he can get China to back down. But if he doesn't then unfortunate things might happen.
Heck, everyone was running scared about the Syrian no-fly zone, I'm not sure how poking China is that much better.


EDIT: Never mind, forget it, I'll stop now.