In 2001, a Vietnam-era student radical named David Horowitz decided to once again start causing trouble on campus. A few years earlier, several scholars and activists had begun to argue that the U.S. should pay reparations to descendants of slaves. Horowitz, whose politics had taken a sharp right turn since the 1960s, thought that this was a very bad idea. So he contacted several college newspapers, seeking to place a full-page ad, during Black History Month, titled, “Ten Reasons Why Reparations for Slavery Is a Bad Idea—and Racist Too.” The ad, which had been adapted from a Salon column he...