By the 2000s, as the internet facilitated more transatlantic dialogue, some American activists had adopted the name antifa. But even on the militant left, the movement didn’t occupy the spotlight. To most left-wing activists during the Clinton, Bush, and Obama years, deregulated global capitalism seemed like a greater threat than fascism.

Trump has changed that. For antifa, the result has been explosive growth. According to NYC Antifa, the group’s Twitter following nearly quadrupled in the first three weeks of January alone. (By summer, it exceeded 15,000.) Trump’s rise has also bred a new sympathy for antifa among some on the mainstream left. “Suddenly,” noted the antifa-aligned journal It’s Going Down, “anarchists and antifa, who have been demonized and sidelined by the wider Left have been hearing from liberals and Leftists, ‘you’ve been right all along.’ ” An article in The Nation argued that “to call Trumpism fascist” is to realize that it is “not well combated or contained by standard liberal appeals to reason.” The radical left, it said, offers “practical and serious responses in this political moment.”

Those responses sometimes spill blood. Since antifa is heavily composed of anarchists, its activists place little faith in the state, which they consider complicit in fascism and racism. They prefer direct action: They pressure venues to deny white supremacists space to meet. They pressure employers to fire them and landlords to evict them. And when people they deem racists and fascists manage to assemble, antifa’s partisans try to break up their gatherings, including by force.

Such tactics have elicited substantial support from the mainstream left. When the masked antifa activist was filmed assaulting Spencer on Inauguration Day, another piece in The Nation described his punch as an act of “kinetic beauty.” Slate ran an approving article about a humorous piano ballad that glorified the assault. Twitter was inundated with viral versions of the video set to different songs, prompting the former Obama speechwriter Jon Favreau to tweet, “I don’t care how many different songs you set Richard Spencer being punched to, I’ll laugh at every one.”

The violence is not directed only at avowed racists like Spencer: In June of last year, demonstrators—at least some of whom were associated with antifa—punched and threw eggs at people exiting a Trump rally in San Jose, California. An article in It’s Going Down celebrated the “righteous beatings.”

Antifascists call such actions defensive. Hate speech against vulnerable minorities, they argue, leads to violence against vulnerable minorities. But Trump supporters and white nationalists see antifa’s attacks as an assault on their right to freely assemble, which they in turn seek to reassert. The result is a level of sustained political street warfare not seen in the U.S. since the 1960s. A few weeks after the attacks in San Jose, for instance, a white-supremacist leader announced that he would host a march in Sacramento to protest the attacks at Trump rallies. Anti-Fascist Action Sacramento called for a counterdemonstration; in the end, at least 10 people were stabbed.