The United States' One-China policy was first stated in the
Shanghai Communiqué of 1972: "the United States acknowledges that Chinese on either side of the Taiwan Strait maintain there is but one China and that Taiwan is a part of China.
[3] The United States does not challenge that position."
This is revisionism. The same kind of "hang on, we never had a formal paper which said NATO cannot expand to the East, it was only an agreement". These agreements were made for specific reasons at specific times. To bring China into the US fold, or closer to it, and away from the Soviet Union. To pretend that the US does not acknowledge that which is bread and butter in US-Chinese relations, or only acknowledges it as if that makes no difference, is to be a complete revisionist in historical terms.
The US knew and knows what that document meant to the Chinese. To try and say "ah, they only said yes to something vague" is completely degenerate (a degenerate understanding of politics, geopolitics, and history: an ideological understanding of such).
Even so, the US officially acknowledged that Taiwan is part of China. How can China invade China? By your own rhetoric it would be a civil war, not an external war. Neither of which is likely to happen any time soon anyway.