"Study the world!" brayed Trump on Twitter last week, in defense of his travel ban. Dan Drezner, who studies the world for a living, shot back: "I have studied it, and I can tell you with some certainty that your words and actions have harmed US national security." On Facebook, I've seen friends and colleagues with PhDs in international relations post their own versions of Dan's tweet: Andrew Moravcsik: "I have studied the world, and people who...




