Believing that a country is in bad shape if it imports more goods than it exports — i.e., if it has a trade deficit — is, in most cases, a harmless error in reasoning. It’s the sort of thing that seems to make sense at first glance, but any halfway decent economics professor can train it out of students in one or two lectures. That error in reasoning becomes harmful when the person who believes it is the president of the United States, and he is willing to claim emergency powers to act on it unilaterally. That’s what Donald Trump...