Andy Campbell has produced a smart, well-written and brilliantly reported book about another loathsome progeny of the most dangerous union of our time, the horror couple responsible for so many of the burgeoning threats to American democracy: Donald Trump and the internet.

Its subject is the Proud Boys, racist, beer-addled and violence-addicted street fighters who have become best friends with many of Trump’s warmest supporters, from Ann Coulter to Roger Stone.

Coulter and Stone have both bragged about using these modern Brown Shirts as bodyguards. Stone even allowed himself to be filmed for a video in which he took the Proud Boys oath: “I’m a western chauvinist. I refuse to apologize for creating the modern world.”

Coulter credited the group with saving her life when “2,000 antifa”, leftwing protesters, tried to shut down a speech at UC Berkeley. If she hadn’t invited 20 Proud Boys, she said, she “might not have made it to the campus at all”.

The Proud Boys are “brawny, tattooed brutes”, Coulter cooed.

As Campbell puts it, the Proud Boys have “proven that you can make it as a fascist gang of hooligans in this country, as long as you make the right friends”.

The organization’s father is Gavin McInnes, 52, a child of Scots who moved to Canada. In Montreal in the early 1990s McInnes founded a magazine called Pervert, which in 1999 he and two others rebranded as Vice. He moved the magazine to New York a couple of years later, then left in 2008.

In spring 2016, on his own talkshow, he declared his main priority: “I want violence. I want punching in the face. I’m disappointed in Trump supporters for not punching enough.”

Not long after that, he “announced that he’d turned his audience into a gang”. He called them the Proud Boys.

McInnes’s alliance with the GOP warmed up after he was invited to speak at the headquarters of the New York state Republican party in October 2018.

Members were undaunted when their intended guest announced on Instagram that he planned to reenact an “inspiring moment … the political assassination of Inejiro Asanuma, the former leader of the Japan Socialist party, who was killed during a debate on live TV when a far-right ultranationalist rushed the stage and pushed a sword between his ribs”.

Then he photoshopped an image of himself “with the eyes and clothing of the Japanese assassin”.

Republicans loved it. On Facebook, they responded: “This Godfather of the Hipster Movement has taken on and exposed the Deep State Socialists and stood up for Western Values. Join us for an unforgettable evening with one of Liberty’s Loudest Voices.”

After his speech, McInnes left the club with his sword. But Proud Boys “and their skinhead pals” attacked a handful of antifascist protesters after one knocked a MAGA hat from one of their heads.

“They turned it into a pummeling,” a Huffington Post reporter remembered. “This was three people on the ground and people just kicking the shit out of them.”

The two most violent attackers were each sentenced to four years in prison. The judge didn’t hesitate to draw the appropriate parallel to 1930s Germany. Mark Dwyer, of the New York state supreme court, said he knew what had happened then, “when political street brawls were allowed to go ahead without any type of check from the criminal justice system. We don’t want that to happen in New York”.

Regardless, the New York brawl became another opportunity for the Republican establishment to normalize fascist behavior. Immediately after the attack, Fox News quoted Ed Cox, the Republican state chairman (and son-in-law of Richard Nixon) as “calling on Democrats to cease inciting these attacks”.

As Campbell writes, the event at the Republican club was “a jumping-off point for the GOP into what would eventually become a full embrace of domestic extremist violence”.

Kelly Weill, a reporter who covers domestic extremism, explained, the Proud Boys “really embody the political violence the GOP needs just a little bit of a proxy for. They can’t personally be out there doing it, so they have the Proud Boys”.

It only took two more years for the Proud Boys to get an official, nationally televised presidential imprimatur, after Joe Biden suggested during a 2020 debate that they were one of the groups Trump should have denounced long ago. Trump said: “Proud Boys, stand back and stand by.”

Henry “Enrique” Tarrio, a former FBI informant and convicted felon who had become the Proud Boys chairman, described the effect of Trump’s declaration.

“We got mentioned, and my life has not been the same since,” Tarrio told Campbell. “My phone started blowing up off the hook. I had 10 fucking news trucks at my house the next morning. I didn’t sleep for … two days.”

Trump’s longtime attorney, Michael Cohen, who turned on his former boss after pleading guilty to charges related to tax evasion and lying to Congress, was sure the president made his statement on purpose.

“If you look at who the Proud Boys really are,” said Cohen, “they’re an army. This is Trump’s army … and when he loses he’s going to use them to try and keep control of power.”

Which of course is what happened. Proud Boys were some of the most active players when Trump urged the crowd in front of him on 6 January 2021 to march on the US Capitol.

Thirteen months after the deadly attack, the Republican endorsement of fascist violence became official: the Republican National Committee unanimously approved a resolution which memorialized the Capitol attack as nothing more than “legitimate political discourse.”

Campbell’s book provides an indispensable account of exactly how the Grand Old Party reached that disgraceful destination.

• We Are Proud Boys: How a Right-Wing Street Gang Ushered in a New Era of American Extremism is published in the US by Hachette