Over the course of Donald Trump’s presidency, the far-right fringe became a surprisingly visible and influential force in American politics. Eruptions of extremist violence — including the Unite the Right rally in Charlottesville, Va., in 2017 and the Jan. 6 Capitol insurrection — have made militant groups like the Proud Boys and conspiracy theories like QAnon into household names. On his popular cable news show, Tucker Carlson recently name-checked the “great replacement” conspiracy theory. And in a recent survey, nearly a third of Republicans agreed with the statement that “true American patriots may have to resort to violence in order to save our country.” The historian Kathleen Belew has spent her career studying political violence and the once-fringe ideas that now animate even right-of-center politics and news media. She is a co-editor of “A Field Guide to White Supremacy” and the author of “Bring the War Home: The White Power Movement and Paramilitary America,” which tells the story of how groups — including the Ku Klux Klan, neo-Nazis and Aryan Nations — coalesced into a radical white-power movement after the Vietnam War. These groups were united by a core set of beliefs about the threats of demographic change and governmental overreach, perceived hostility toward white Americans and the necessity of extra-political, often violent, action to achieve their aims. This is a conversation about how some of those ideas have seeped into mainstream Republican politics and what that could mean for the future of the party — and the country. It explores the radicalizing effects of Jan. 6, how irony and meme culture import far-right ideas into popular media, how warfare abroad can produce violence at home, why politics has started to feel apocalyptic across the spectrum, whether left-wing violence is as serious a threat as right-wing violence and more.