Mr. Biden’s wording was a reminder of what a minefield Taiwan remains for the United States, 42 years after the passage of the Taiwan Relations Act and amid a major buildup of Chinese military forces in the region. And once a strategy of ambiguity is described in less-than-ambiguous terms, as he seemed to do on Thursday, it is hard to walk it back.

Mr. Biden is hardly new to the issue: He is one of the very few political figures who have been around Washington so long that he voted for the act, in 1979, as a young senator from Delaware. As chairman of the Senate Foreign Relations Committee, he traveled to Taiwan and understood the nuances of the wording.

He understood it so well, in fact, that 20 years ago Mr. Biden warned President George W. Bush that “words matter” after Mr. Bush said he would do “whatever it took” to defend Taiwan. When, a few hours later, the Bush White House did what the current White House did, saying that nothing had changed, Mr. Biden wrote an opinion column correcting him, noting that “the United States has not been obligated to defend Taiwan.”