The Taiwan-American defense treaty was abrogated in 1979 when Jimmy Carter and both houses of Congress sold out to Communist China, broke diplomatic relations with Taiwan, and recognized Communist China instead.
It was replaced with the Taiwan Relations Act, which is not a full mutual defense pact. On the other hand it is not insignificant either. In time of peril, such as now, its strength will depend on the interpretation of the president at the time. The presidency of Donald Trump has been favorable to strengthening Taiwan, but its commitment in case of open hostilities is unknown.
What if we restored the treaty?