Our ground forces are occupied in Iraq. Our Navy and air assets are what would stop them from taking Taiwan. Those assets are not tied down in Iraq to any appreciable extent.
That said, I'm not sure we could get together the political will to defend Taiwan. The lib pacifists and appeasers might well let it go.
Attack the US proper or Japan or S. Korea, and China will find out why we are acknowledged the world's only superpower. And those that are so certain we'll never use nukes are living in a dream world. MAD is still a working policy, and Americans still understand only two choices exist: live free or die.
If there is a war between the US and China, it will quickly go nuclear.