This past September 11, a group of activists gathered at Bluestockings, a cooperatively owned bookstore in lower Manhattan, to learn about fighting fascism in Trump’s America. Some of them were members of "antifa," a loose network of activists whom the president had just called out as a threat to the country.

Antifa members have been known for starting fires, punching Nazis, and donning balaclavas. At least one person at the bookstore wore a mask. Shelves bearing labels like "anarchism" and "feminist masculinity" were rolled out of the way to make room for the crowd, but there wasn’t enough space for all the people who wanted to hear Mark Bray speak.

Mr. Bray, a 35-year-old who looks even younger than that, is neither a fire-starter or a Nazi-puncher. He is a lecturer of history at Dartmouth College. His weapons of choice are logic and history, his crisp sentences delivered with the vaguest trace of a New Jersey accent.

He came here to give a lecture, not lead a rally. Yet the audience hung on his words, grateful to a man who has spent the better part of a year becoming a student and teacher of the antifascist movement — and a de facto translator for mainstream liberals wondering what kind of "resistance" they are willing to justify.

Bluestockings was Mr. Bray’s first appearance on a 35-stop tour to promote Antifa: The Anti-Fascist Handbook (Melville House), a book he’d never planned to write. He had researched turn-of-the-century Spanish radicalism as a doctoral student at Rutgers University at New Brunswick, and seemed well on his way to a life of teaching undergraduates and writing about modern European history. Then Donald Trump won the presidency, white nationalists rejoiced, and 20th-century European fascism was suddenly on everyone’s mind.

An invitation to talk about antifascism on National Public Radio yielded the book deal — then more radio interviews, multiple appearances on Meet the Press, and a Washington Post op-ed. It also yielded death threats and a rebuke from Dartmouth’s president, who was asked why Mr. Bray had, in an interview with MSNBC, appeared to offer a defense of violent tactics while standing in front of a backdrop wallpapered with the Ivy League university’s name. ...