The mutual defense treaty with the ROC (Taiwan)-
www.yale.edu/lawweb/avalon/diplomacy/china/chin001.htmwas ended after a 12 month notification as part of the conditions Red China layed down for relations.
In order to continue with "business as usual" without the usual, Congress passed legislation to protect Taiwan.
The Taiwan Relations Act was passed by Congress and singed into US law.
www.wufi.org.tw/eng/feldman.htmThis is an act, passed by Congress and signed by the President into law that requires the USA to maintain peace in the region and specifically protect the peace of Taiwan. It is the only nation to have protection of US law, not just treaty. It is also not a nation in the sense of relations with the US. Military equipment, training, and economics still go on as though it were a recognized nation, just that they aren't.
The act is what's pretty much prevented any president from just haning Taiwan out to dry, which no doubt would have been the easy route to take with some of them.
The recent views of China becoming a threat or competitor, and Japan's trouble with them and it's pledge to defend Taiwan with the USA has complicated US/China relations. In the end it seems as though everyone is actually getting what they want out of the deals, so it's unlikely any force would be used to take Taiwan.
China is getting everything it wants handed to it on a silver platter right now. There's no reason to change that unless Taiwan declares independence. That declaration would be impossible for China to take laying down. They've pretty much painted themselves into a corner on that one. With the economic ties the USA has with both, pretty much everyone gets fucked if war happens. The end state is the US sits on a fence it made and maintains between the two. One thing the Chinese have is patience, and as long as it looks like nothing's going bad, they won't start anything. Taiwan and China have economic ties as well going on, and that is another thorn in the side of an easy solution.
China will push every president to see where he stands. One thing that really bugs China is having to reinvent it's relations with the USA every four to eight years. But as long as no one on Taiwan does anything stupid, nothing is going to happen. Military action is frankly remote.