They turned out by the hundreds at Trump’s inauguration in January, vandalizing a Starbucks and a Bank of America and torching a limousine in downtown Washington.

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And on Wednesday, swarms of them shut down a planned speech by the conservative provocateur and Breitbart writer Milo Yiannopoulos at the University of California at Berkeley. Some 1,500 people had showed up to demonstrate against the event when a black-clad mob of a few dozen started breaking windows and setting fires at the campus.

Law enforcement agencies and journalists are often quick to dub such groups “anarchists” — and for the most part that’s accurate. But the practice of activists dressing in black en masse has its own unique origins and aims.

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It’s called “black bloc,” and it’s a tactic — sometimes mistaken for an organization or a movement — that protesters have long used to give themselves anonymity at demonstrations, to achieve both violent and nonviolent ends.

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“By putting on our masks we reveal our unity; and by raising our voices in the street together, we speak our anger at the facelessness of power,” reads a popular anarchist credo that was printed on the inside of masks distributed at a violent anti-capitalist protest in London in 1999.

Police Magazine, as you might expect, has a somewhat different impression of “black bloc.”

“In the ‘Black Bloc’ stratagem, throngs of criminal anarchists all dress in black clothing in an effort to appear as a unified assemblage, giving the appearance of solidarity for the particular cause at hand,” the magazine wrote in January 2015. “This tactic is particularly troubling for law enforcement security forces, as no anarchist rioter can be distinguished from another, allowing virtual anonymity while conducting criminal acts as a group.”

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An oft-cited history of “black bloc” tactics by Daniel Dylan Young of A-Infos, a multilingual anarchist news and information service, suggests that the practice has its roots in Germany in the late 1970s. At the time, hordes of young people had taken residence in vacant buildings in inner cities, setting up cooperative houses in the bowels of abandoned warehouses and tenements. Similar communities cropped up in the Netherlands, Denmark and elsewhere in Northern Europe.

In 1980, however, the city governments began to crack down. German authorities evicted and arrested thousands of squatters that winter, triggering protests across the country, one of which turned violent in Berlin, with rioters destroying an upscale shopping area, according to Young.

“In response to violent state oppression radical activists developed the tactic of the Black Bloc,” Young wrote in 2001. By masking up in black, he wrote, activists “could more effectively fend off police attacks, without being singled out as individuals for arrest and harassment later on.”

The tactic spread to Amsterdam and other cities with large squatter populations. Toward the end of the decade, protesters were making wide use of it. In summer 1987, when President Ronald Reagan delivered his famous “tear down this wall” speech in West Berlin, he was met by tens of thousands of protesters, including a 2,000-person “black bloc,” as the New York Times reported then.

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It’s not clear exactly when “black bloc” tactics crossed the Atlantic, but two large protests in 1990 — one in Washington against the Gulf War, the other in San Francisco against Columbus Day — were both disrupted by black-clad groups that destroyed downtown property, according to Young.

The tactic was hardly ever more visible than it was during the massive protests against the 1999 World Trade Organization summit in Seattle. Demonstrations began peacefully, but several hundred “black bloc” activists — described by the Seattle Post-Intelligencer at the time as “masked anarchists wearing black” — smashed windows, looted stores and vandalized buildings. The confrontation, dubbed the “Battle in Seattle,” delayed the start of the meeting and cast a shadow over the proceedings.

Young said those protests cemented the tactic’s use in the United States.

“The Black Bloc in Seattle inspired a renewed interest in militant protest tactics which do not placate authority or bow to its power,” he wrote. The protests there, he added, “also inspired radical anarchists to stop hiding out inside liberal activist groups with reformist agendas, and start being more vocal in their demands for revolution and total social change.”

For years now, masked activists in black have been a fixture at demonstrations nationwide — from Black Lives Matter rallies to political conventions to Occupy Wall Street. On Wednesday, they destroyed ATMs, broke windows in the student union and set a fire in the campus plaza at Berkeley, prompting a campus lockdown.

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From the black bloc-ers perspective, property destruction is all about attacking the symbols of capitalism and corporate greed. Some don’t even view it as violent.

“We contend that property destruction is not a violent activity unless it destroys lives or causes pain in the process,” read a “black bloc communique” from the Seattle protests in 1999. “When we smash a window, we aim to destroy the thin veneer of legitimacy that surrounds private property rights.”

But for demonstrators seeking to protest peacefully, “black bloc” activity is at best a distraction. The writer and activist Devon Douglas-Bowers has argued that “black bloc” protesters “fetishize” property destruction at the expense of nonviolent action, and the popular left-wing journalist Chris Hedges once called them the “cancer of the Occupy movement.” Beyond that, go to any large rally, and the calls for peace almost always drown out the calls for destruction, even if they don’t stop it.

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Law enforcement officials acknowledge the difference, too.

“What they’re doing is they’re taking advantage of the legitimate protesters to destroy things and emphasize their anarchist roots,” David Gomez, a former senior FBI counterterrorism official in Seattle, told The Washington Post after the anti-Trump protests in November.

Young, the author of the “black bloc” history, questioned the future of the tactic in his 2001 essay for A-Infos.

“It is important that we neither cling to it nostalgically as an outdated ritual or tradition, nor reject it wholesale because it sometimes seems inappropriate,” he wrote. “Rather we should continue working pragmatically to fulfill our individual needs and desires through various tactics and objectives, as they are appropriate at the specific moment.”