The US used to have tens of thousands of troops in Taiwan. The US military formally withdrew in 1979, although it has quietly retained small numbers of US military personnel in Taiwan ever since, responsible for training and liaison with Taiwanese military forces, but not enough to be militarily significant.

A quick, unexpected, massive deployment of significant US military assets to Taiwan would put Beijing in a very difficult position. Either Beijing attacks, and starts a shooting war with the US – which would do massive damage to the Chinese economy; or else, Beijing doesn't and loses a lot of face in the process.

It would be a rather risky, high stakes gamble, but one in which the US might come out in front.