(Doug Mills | The New York Times) President Joe Biden listens to a reporter's question as as he and Prime Minister Fumio Kishida of Japan held a joint news conference in Tokyo, May 23, 2022. This is not the first time Biden has suggested the United States would fight for Taiwan, but the last time he said something along similar lines, it was treated as a classic Biden gaffe. Now it should be clear he means it, Bret Stephens writes.