As a private in the Florida National Guard, Brandon Russell led a double life, carrying out exercises with his infantry team while heading a violent neo-Nazi group and reportedly recruiting like-minded troops until 2017.

At his condo in Tampa, he had a flag with the Nazi swastika on the wall, a framed photo of Oklahoma City bomber Timothy McVeigh — himself a military veteran — and a cache of bomb-making material in the garage.

The attempted insurrection at the Capitol on January 6 — after which veterans and active-duty service members accounted for at least 27 of the initial arrests, or nearly 20 per cent of the total — exposed the crossover of military personnel to violent extremism and White supremacy. The participants included a former Navy Seal and a retired Air Force lieutenant-colonel, photographed on the Senate floor wearing tactical gear and carrying zip-tie handcuffs. The military was overrepresented among the rioters.

The ugly sight of service members in combat gear amid Confederate flags and White-nationalist slogans marked a harrowing escalation of a problem that the Pentagon has allowed to fester for decades. It confirmed the worst fears about an extremist subculture in the armed services acting out of loyalty not to their sworn oaths but to their own dangerous agenda. It should have surprised no one.

Ronald Reagan's first defence secretary, Caspar Weinberger, pledged to combat extremists in the military in 1986 after reports showed that numerous service members were active in the Ku Klux Klan and other White-supremacist groups. He banned military personnel from having any involvement with such organisations, and as reports have continued to emerge in the years since, Pentagon officials have issued numerous follow-up restrictions. Indeed, the Pentagon had ordered up two more reviews of the problem less than a month before the Capitol attack. Still, the pull of extremism and White supremacy in the ranks has persisted — a reflection, some outside experts say, of poor enforcement and oversight by the military of its own rules.

Take Russell, the neo-Nazi leader in Florida. When he entered the National Guard, screeners took note of a tattoo on his shoulder of a radiation-warning symbol, but they didn't recognise it as the logo of his group, the Atomwaffen Division. While officers reprimanded Russell for his vocal homophobia in the barracks, they said there were no clear signs at the time of his dual life as a neo-Nazi — although prosecutors later charged that he had used his status and training in "fulfilling his neo-Nazi mission".

It wasn't until a double killing at Russell's condo in Tampa in May 2017 — when another man who had been involved with Atomwaffen allegedly shot two other members — that police discovered the huge stockpile of explosives Russell had in his garage, along with firearms and his stash of neo-Nazi paraphernalia. As police searched the scene, Russell was still dressed in his Guard uniform after returning home from weekend duty. He is now in federal prison serving a five-year sentence on explosives charges.

Days before President Joe Biden's inauguration on January 20, concerns about internal security prompted an eleventh-hour vetting of the 25,000 National Guard troops massed in the capital to protect the swearing-in. The background checks led the Guard to remove a dozen members from the inauguration mission — all of whom, according to the Associated Press, "were found to have ties with right-wing militia groups or posted extremist views online". The military has not explained why those ties were not discovered before the personnel were sent to protect the event.

Since the Capitol riot, military officials have vowed to redouble their efforts to weed out extremists and White supremacists. But there is little to suggest that such steps represent more than lip service in combating such a pervasive problem, especially as the racial chasm in America grew wider during the Trump era.

Derek Barsaleau was an Army specialist who served in Afghanistan, but he also kept up a proud allegiance to neo-Nazi groups, including the notorious National Alliance and the World Church of the Creator. He was never shy about voicing his beliefs to his fellow soldiers. The military "was rampant with White supremacists", Barsaleau told me. Since then he has denounced that ideology and now works with an anti-hate group in Wisconsin. But he says the appeal of White supremacy to service members has only become more powerful in the past few years — thanks in part, he believes, to Donald Trump's divisive racial rhetoric.

Indeed, a survey published last year by the Military Times found that 36 per cent of active-duty service members — and half of minorities — had seen signs of White nationalism or ideologically driven racism in the armed forces, a spike of 14 percentage points from just a year earlier. Remarkably, service members also reported that they considered White nationalism within their own ranks to be a greater national security threat than domestic Islamic terrorism — and more worrisome than North Korea, Afghanistan, Iraq or other global threats.

Warning signs abounded just last year in a string of violent, politically inspired episodes carried out by active-duty or retired service members, several in direct response to Black Lives Matter protests. There was the killing of a security officer and the wounding of another near a BLM protest in Oakland, California — allegedly by an active-duty Air Force sergeant who had aligned himself with the anti-government "boogaloo boys" movement. In Las Vegas, three veterans also affiliated with the boogaloos were charged with plotting to detonate Molotov cocktails at a BLM protest and to stoke violence among protesters. And a 22-year-old Army private from Louisville was accused of plotting with an overseas neo-Nazi group to attack his own military unit in Turkey. A federal prosecutor declared that the soldier, Ethan Melzer, "was the enemy within".

Meanwhile, anti-government extremist groups such as the Three Percenters and the Oath Keepers have made inroads in recruiting present and former members not only of the military, but of police agencies across the country. Both groups had a presence at the Capitol riot.

Predictably, in the wake of the failed insurrection, Pentagon officials have denounced the role of their own personnel. "These people are not representative of our country's military," General Mark A. Milley, chairman of the Joint Chiefs of Staff, told The New York Times. Whether the military can make good on its oft-repeated pledges to rid itself of such extremists is an urgent dilemma confronting the entire country.

• Eric Lichtblau is a journalist and author who writes about national security and law enforcement. He is writing a book on the surge in hate crimes carried out by White supremacists