The answer is simple: The US is the Global Hegemon of our times, and furthermore, it is a formal guarantor of Taiwan's security (this is similar in many ways to

The answer is simple: The US is the Global Hegemon of our times, and furthermore, it is a formal guarantor of Taiwan's security (this is similar in many ways to its allies in NATO). Economically, Taiwan and the US have deep ties: it would make sense for the US to come to Taiwan's aid for this reason alone. Whether the ally is across the Pacific or elsewhere scarcely matters to the US, because it has power projection capability that spans the entire planet. That is doubly so when you consider that it is allied with almost all the major developed countries and almost every single one of China's neighbors (Japan, South Korea, the Philippines, and to a lesser extent Vietnam and India). If China were the hegemon in the 1800s, then of course it would have intervened in the Civil War. Take France, which was a World Power (and traditional rival of Great Britain) during the War for Independence. It did provide support to the US, and was their primary ally.