Ava DuVernay is a film-maker who has long been committed to a bracing cinema of ideas. Her 2016 documentary, 13th, was an investigation of the incarceration of black men and the constitutional mindset link between imprisonment and slavery in the US. It was to this film, I think, that we owed the larger revelation about the civil war: the Confederacy was not defeated like Germany was in 1945, but in fact like Germany had been in 1918 at the end of the previous world war – provisionally, and unstably. Now she has turned to the journalist and commentator Isabel Wilkerson,...