The United States has no military obligations toward Taiwan, it doesn't even have formal diplomatic relations with the country. I could see the United States intervening to 'save' Japan but not Taiwan.
You are correct about the “legal” relationship between the US and Taiwan, but unofficially the Americans have always made it clear to China that it would not tolerate a Chinese invasion of Taiwan. The US regularly sends its warships between mainland China and Taiwan as a symbolic gesture and the US has helped with Taiwanese military defences (although not to the extent that Taiwan wishes). My point is that the Chinese always had significant uncertainty as to what the US would do if it invaded Taiwan. The situation in Crimea is making it crystal clear to Beijing that the US will do nothing to assist Taiwan if China makes its play. Russian and Chinese expansionism is not good for anyone.