If China were to attack Taiwan, would American forces come to the island’s defense? It is hard to know because the U.S. maintains a policy of “strategic ambiguity” concerning how it would respond. It’s time for that to change.

The Taiwan Relations Act of 1979 states only that the U.S. would regard such an attack as of “grave concern,” and only commits the U.S. to maintaining the ability to defend the island. This is a much less firm commitment than the U.S. offers in NATO and to allies Japan and South Korea. But the U.S. stakes...