People who want to compare divisiveness in the U.S. today with pre-Civil War America haven't read enough antebellum history. And people who would hold up 1930s Nazi Germany as the comparison model seem to have not read any history at all. What today feels more like, with cultural and political differences defined most along urban-rural delineations and tensions high between protesters and police, is the mid-to-late 1960s. In what became known as the "long hot summer of 1967," more than 150 riots broke out across American cities. This widespread rioting followed more isolated incidents in Harlem in New York in...