The historian Robert Paxton spent Jan. 6, 2021, glued to his television. Paxton was at his apartment in Upper Manhattan when he watched a mob march toward the Capitol, overrun the security barriers and then the police cordons and break inside. Many in the crowd wore red MAGA baseball caps, while some sported bright-orange beanies signaling their membership in the Proud Boys, a far-right extremist group. A few were dressed more fantastically. Who are these characters in camouflage and antlers? he wondered. “I was absolutely riveted by it,” Paxton told me when I met him this summer at his home in the Hudson Valley. “I didn’t imagine such a spectacle was possible.”

Paxton, who is 92, is one of the foremost American experts on fascism and perhaps the greatest living American scholar of mid-20th-century European history. His 1972 book, “Vichy France: Old Guard and New Order, 1940-1944,” traced the internal political forces that led the French to collaborate with their Nazi occupiers and compelled France to reckon fully with its wartime past.

The work seemed freshly relevant when Donald Trump closed in on the Republican nomination in 2016 and articles comparing American politics with Europe’s in the 1930s began to proliferate in the American press. Michiko Kakutani, then the chief book critic for The New York Times, was among the first to set the tone. She turned a review of a new Hitler biography into a thinly veiled allegory about a “clown” and a “dunderhead,” an egomaniac and pathological liar with a talent for reading and exploiting weakness. In The Washington Post, the conservative commentator Robert Kagan wrote: “This is how fascism comes to America. Not with jackboots and salutes,” but “with a television huckster.”

In a column for a French newspaper, republished in early 2017 in Harper’s Magazine, Paxton urged restraint. “We should hesitate before applying this most toxic of labels,” he warned. Paxton acknowledged that Trump’s “scowl” and his “jutting jaw” recalled “Mussolini’s absurd theatrics,” and that Trump was fond of blaming “foreigners and despised minorities” for ‘‘national decline.’’ These, Paxton wrote, were all staples of fascism. But the word was used with such abandon — “everyone you don’t like is a fascist,” he said — that it had lost its power to illuminate. Despite the superficial resemblances, there were too many dissimilarities. The first fascists, he wrote, “promised to overcome national weakness and decline by strengthening the state, subordinating the interests of individuals to those of the community.” Trump and his cronies wanted, by contrast, to “subordinate community interests to individual interests — at least those of wealthy individuals.”

After Trump took office, a torrent of articles, papers and books either embraced the fascism analogy as useful and necessary, or criticized it as misleading and unhelpful. The polemic was so unrelenting, especially on social media, that it came to be known among historians as the Fascism Debate. Paxton had, by this point, been retired for more than a decade from Columbia University, where he was a professor of history for more than 30 years, and he didn’t pay attention to, let alone participate, in online debates.