On Thursday 11 April, shortly after 11pm, a black Columbia student named Alexander McNab walked through the gates of Barnard college—the undergraduate all-women’s school at Columbia University—after ignoring a security guard’s request to show his student ID. In search of a midnight snack, McNab got all the way to the library canteen before a public safety officer confronted him and asked for his ID a second time, a request McNab once again refused. Several more officers had arrived on the scene and were continuing to request ID when McNab began yelling. What happened next, depicted in the video below, has...