Stewart Rhodes, the founder of the rightwing Oath Keepers militia, has been found guilty of seditious conspiracy, a charge arising from the attack on the US Capitol by supporters of Donald Trump.

Rhodes and co-defendant Kelly Meggs are the first people in nearly three decades to be found guilty of the rarely used civil war-era charge at trial. The trial was the biggest test yet for the US justice department in its efforts to hold accountable those responsible for the attack that shook the foundations of US democracy.

On social media, Harry Litman, a former US attorney turned legal analyst, said the guilty verdicts represented “a huge huge victory for the US [justice department] in a challenging and deeply important, even historic, case”.

Rhodes is a Yale Law-educated former paratrooper and disbarred attorney. In an eight-week trial, he and four associates were accused of fomenting a plot to use force to stop Congress from certifying Joe Biden’s 2020 election victory.

It was the most significant trial arising from the Capitol riot of 6 January 2021, which has been linked to nine deaths including suicides among law enforcement officers. A US district judge, Amit Mehta, presided. The 12-member jury deliberated for three days.

Rhodes’ four co-defendants were Meggs, Kenneth Harrelson, Jessica Watkins and Thomas Caldwell.

Meggs was convicted of seditious conspiracy. Harrelson, Caldwell and Watkins were acquitted.

During the trial, Watkins admitted impeding police officers, and apologized. All five defendants were convicted of obstruction of an official proceeding, with mixed verdicts on a handful of other charges. Rhodes was acquitted of two other conspiracy charges.

Rhodes intends to appeal, defense attorney James Lee Bright told reporters. Another Rhodes lawyer, Ed Tarpley, described the verdict as a “mixed bag”, adding, “This is not a total victory for the government in any way, shape or form.”

“We feel like we presented a case that showed through evidence and testimony that Mr Rhodes did not commit the crime of seditious conspiracy,” Tarpley said.

Rhodes, who wears an eye patch after accidentally shooting himself in the face, was one of the most prominent defendants of around 900 charged so far in connection with the Capitol attack.

He founded the Oath Keepers, whose members include current and retired military personnel, law enforcement officers and first responders, in 2009. Members have showed up, often heavily armed, at protests and political events including demonstrations following the May 2020 murder of George Floyd by a white police officer in Minneapolis.

Prosecutors said Rhodes and his co-defendants planned to use force to stop Congress from certifying Biden’s win.

Rhodes did not go inside the Capitol but was accused of leading the plot. Through recordings and encrypted messages, jurors heard how he rallied followers to fight to keep Trump in office, warned of a “bloody” civil war and expressed regret that the Oath Keepers did not bring rifles on 6 January.

Meggs, Watkins and Harrelson entered the Capitol wearing tactical gear. The defendants were accused of creating a “quick reaction force” positioned at a Virginia hotel and equipped with firearms that could be quickly transported to Washington.

Fifty witnesses testified. Rhodes and two others testified in their own defense. They denied plotting an attack or seeking to stop Congress from certifying results. Rhodes insisted that his followers who went inside went rogue.

Prosecutors sought to paint Rhodes as a liar, showing him his own inflammatory text messages, videos, photos and recordings. These included Rhodes saying he could have hanged the House speaker, Nancy Pelosi, from a lamppost.

Watkins, a transgender woman who fled the US army, and Caldwell, a disabled navy veteran, were the others who chose to testify.

Watkins admitted “criminal liability” for impeding officers inside the Capitol but denied any plan to storm the building, instead describing being “swept up” in the moment, just as enthusiastic shoppers behave when they rush into stores to purchase discounted holiday gifts.

Caldwell, who like Rhodes did not enter the Capitol, never formally joined the Oath Keepers. He tried to downplay inflammatory texts he sent in connection with the attack, saying some of the lines were adapted from or inspired by movies such as The Princess Bride or cartoons such as Bugs Bunny.

Four other Oath Keepers members charged with seditious conspiracy are due to go to trial in December. Members of another rightwing group, the Proud Boys, including its former chairman Enrique Tarrio, also are due for trial on seditious conspiracy charges in December.