On Monday afternoon, New York’s Columbia University looked like a military installation, not one of the most storied institutions of learning in the United States. The campus gates, festooned with balloons designed to “welcome” newly admitted students, all were padlocked shut. Columbia’s main, beaux-arts campus occupies more than six city blocks in the Morningside Heights area of Manhattan and around its huge perimeter Monday were stationed security guards about every 10 feet. At one of the main gates on Broadway, a strikingly large phalanx of police stood by in riot gear as protesters, many of them wearing the keffiyeh, the...