The Trump administration’s decision to pause all visa interviews for foreign students who want to study in America, pending a review of how applicants’ social-media posts are vetted, is yet another escalation in the power struggle over who controls the world’s best universities. The policy may be modified. It may prove less onerous than it looks at first glance. Even if that happens, though, this is another blow to a great American success story. President Donald Trump cares about America’s trade deficit. So it is perverse for him to make it harder for one of America’s most prodigious exporters—the education...