“It sounded like we were going to war against the United States government,” a former member of the militia group testified.WASHINGTON — Two days after news organizations called the 2020 election for Joseph R. Biden Jr., Stewart Rhodes, the leader of the Oath Keepers militia, held a video meeting with his members to discuss bringing weapons to a Stop the Steal rally in Washington that month and to urge them to “fight” on behalf of President Donald J. Trump.Listening to the meeting was Abdullah Rasheed, a Marine Corps veteran and a member of the far-right group from West Virginia. During testimony on Thursday at the trial of Mr. Rhodes and four of his subordinates, Mr. Rasheed told the jury that he was so disturbed by what he heard during the meeting that he recorded the conversation and ultimately called the F.B.I. to alert them about Mr. Rhodes.“The more I listened to the call,” he said, “it sounded like we were going to war against the United States government.”The testimony by Mr. Rasheed, a heavy-equipment mechanic, was clearly intended to bolster accusations by the government that Mr. Rhodes and his co-defendants — Kelly Meggs, Kenneth Harrelson, Jessica Watkins and Thomas Caldwell — committed seditious conspiracy by using force to oppose Mr. Biden’s ascension to the White House.The five Oath Keepers are the first of nearly 900 people charged in connection with the Capitol attack on Jan. 6, 2021, to face trial on sedition charges, the most serious crime that prosecutors have brought against any of the defendants.Even as testimony proceeded, in another case in the same Federal District Court in Washington, Jeremy Bertino became the first member of another far-right group, the Proud Boys, to plead guilty to seditious conspiracy charges. As part of a deal with the government, Mr. Bertino agreed to testify against Enrique Tarrio, the Proud Boys’ former chairman, and four other members of the group at their upcoming trial.The Proud Boys trial, scheduled for December, is likely to feature accusations that members of the group were instrumental in several breaches of the Capitol’s defenses and helped to rile up other protesters in assaulting the police. Charles Donohoe, a Proud Boys leader from North Carolina, pleaded guilty to conspiracy and assault charges in April and has been cooperating with the government against others in the group.On Tuesday, prosecutors at the Oath Keepers trial played several clips of Mr. Rasheed’s recording for the jury. The jurors heard Mr. Rhodes make baseless claims about foreign interference in the election and declare that he would welcome violence from leftist antifa activists because that would give Mr. Trump an excuse to invoke the Insurrection Act and call on militias like his own to quell the chaos.“We’re not getting out of this without a fight,” Mr. Rhodes said. “There’s going to be a fight. But let’s just do it smart, and let’s do it while President Trump is still commander in chief.”While Mr. Rasheed initially called an F.B.I. tip line to complain about Mr. Rhodes not long after the meeting took place, the bureau did not reach out to him until March 2021, two months after the Capitol was attacked. He also tried to warn other law enforcement agencies, he testified, writing to the Capitol Police that Mr. Rhodes was “a friggin’ wacko that the Oath Keepers would be better without.”Mr. Rasheed’s turn on the witness stand came between testimony from two other former Oath Keepers: Michael Adams, the onetime leader of the group’s Florida chapter, and John Zimmerman, who once ran a chapter in Cumberland County, North Carolina.Both men told the jury how they became involved with the Oath Keepers amid the street protests in the summer of 2020 out of concern about the leftist antifa movement but eventually, like Mr. Rasheed, became disillusioned by Mr. Rhodes’s extreme rhetoric.Mr. Adams was the administrator of the video meeting that Mr. Rasheed recorded and told the jury that he was put off by Mr. Rhodes’s claims that the election had been stolen as well as by his personal attacks against Mr. Biden, who he repeatedly referred to as a “puppet” of the Chinese Communist Party.The Oath Keepers are accused of plotting to use violence to keep Mr. Trump in office after his defeat at the polls.Kenny Holston for The New York TimesIn December, after Mr. Rhodes wrote two open letters to Mr. Trump, urging the president to invoke the Insurrection Act and call up militias like the Oath Keepers to quell potential violence from the left, Mr. Adams was further disillusioned, he said. Unable to serve with a group that was considering using force in a manner he felt might be illegal, he left the Oath Keepers for good.Mr. Zimmerman, an Army veteran who once ran an emergency preparedness store, told the jury a similar story. He testified that shortly after the so-called Million MAGA March — a pro-Trump rally that took place in Washington in November 2020 — he was horrified when Mr. Rhodes proposed a brazen new plan for going after antifa.Because antifa often attacked the “weak and elderly,” Mr. Zimmerman said, Mr. Rhodes suggested that, at future pro-Trump rallies, the Oath Keepers should sucker their opponents into fighting by dressing up as old people or as parents pushing strollers.“If we could entice them to attack us,” said Mr. Zimmerman, who wore a mask reading “Front Toward Enemy” as he took the witness stand, “then we could give them a beat down.”Much of the testimony Thursday homed in on the ways that antifa loomed large in Mr. Rhodes’s imagination and how the leftist movement has played a central role in the arguments that he and his co-defendants have raised against the charges they used force to stop the certification of the Electoral College vote at a joint session of Congress on Jan. 6.Since the trial began on Monday, lawyers for the Oath Keepers have told the jury that their clients never had a plan to use violence at the Capitol but instead took measures to prepare for violence from antifa, including the creation of an armed “quick reaction force” staged outside Washington in hotel rooms in Virginia.Mr. Zimmerman told the jury that during the Million MAGA March he helped to oversee a similar armed force that was staged at Arlington National Cemetery. Stashed in his oversized RAM ProMaster van, he said, were 12 to 15 assault-style rifles, a half-dozen handguns and several sawed-off pool cues that were to be used in case he needed to rush to the aid of his compatriots in the city.Mr. Rhodes talked incessantly about antifa, Mr. Zimmerman said, telling his fellow Oath Keepers that they had to prepare for violent attacks from the movement even before the election took place. At a pro-Trump rally in North Carolina in September 2020, Mr. Zimmerman recalled, Mr. Rhodes coordinated the militia’s efforts to protect Trump supporters with someone he claimed was in the Secret Service.One of the oddities of the Oath Keepers trial is that few of the facts presented so far are in dispute. That has allowed the defense and prosecution to effectively set aside the question about what the group did on Jan. 6 and the days leading up to it and focus instead on the reasons that they did it.Central to this dispute about intent has been the purpose of the “quick reaction force” and the role that antifa played in Mr. Rhodes’s decision to employ them not only on Jan. 6, but also at the Million MAGA March in November and at another pro-Trump rally in December.Indeed, prosecutors began on Thursday by showing the jury an “ops plan” that Mr. Caldwell, a former naval officer, had drafted in advance of a rally in Washington on Dec. 12, 2020.Fearing there might be trouble from antifa, Mr. Caldwell advised his compatriots in the plan to bring “striking weapons” to the rally, suggesting a specific brand of tomahawk called the “Zombie Killer.”Mr. Caldwell also recommended the militia’s leaders should consider bringing firearms as well, especially those that could not be traced to any members. The guns — and each of their bullets — should be wiped down thoroughly before the event, Mr. Caldwell wrote, and discarded after use.While Mr. Caldwell’s plan clearly stated that it was meant to detail steps for confronting “antifa and other criminal elements,” prosecutors suggested that it showed a proclivity for violence that was ultimately turned on members of Congress.“If we’d had guns I guarantee we would have killed 100 politicians,” Mr. Caldwell wrote that evening in a Facebook message that prosecutors showed the jury Thursday. “They ran off and were spirited away through their underground tunnels like the rats they were.” More