China did Donald Trump a favor this week—by insulting him. In an interview with this newspaper, Chinese Finance Minister Lou Jiwei called the New York businessman an “irrational type” whose trade policies would leave the U.S. no longer “entitled to world leadership.” The Donald boasted in reply: “I’m so glad they’re upset. They haven’t been upset with us in 30 years.” American leaders have let China “abuse” us, he told a Staten Island crowd, but “that’s not going to happen anymore with me.”

A classic Trump promise: clear,...