Taiwan is a sovereign nation in most respects, at least to the extent that any non-superpower is. China insists stringently on the legal fiction that it is a part of China controlled by a different faction in a frozen-but-ongoing civil war, and other countries - not least Taiwan itself - go along with that legal fiction under the implicit or explicit threat of Chinese force; a threat that is less effective against the US than against many other countries.

Is this an act of war by the US against the PRC? Eh, maybe, if you accept the PRC's territorial claims. A lot of technically-acts-of-war happen, especially in situations like this. Again this is a case where the abstraction breaks down and the realpolitik becomes paramount.