When rioters, encouraged by the President, stormed the Capitol, one year ago, to overturn the results of the election, the idea that such a thing could play out in America was stunning. But the attack may have been just the beginning of an ongoing insurrection, not a failed attempt at a coup. David Remnick talks with Barbara F. Walter, the author of the new book “How Civil Wars Start: And How to Stop Them.” Walter is a political scientist and a professor at the University of California, San Diego, and a co-director of the online magazine Political Violence at a Glance. She has studied countries that slide into civil war for the C.I.A., and she says that the United States meets many of the criteria her group identified. In particular, anti-democratic trends such as increased voting restrictions point to a nation on the brink. “Full democracies rarely have civil wars. Full autocracies rarely have civil wars,” she says. “It’s the ones that are in between that are particularly at risk.”