In the wake of the Jan. 6 attack on the U.S. Capitol, academics and journalists have taken the possibility of future political violence in America increasingly serious. In her new book, “How Civil Wars Start: And How To Stop Them,” Barbara F. Walter writes, “We are closer to civil war than any of us would like to believe.” It’s a bold suggestion, and we interrogate it in this installment of the FiveThirtyEight Politics podcast.