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 now 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.