WASHINGTON — The founder of the Oath Keepers extremist group and four associates planned an “armed rebellion” to keep President Donald Trump in power, a federal prosecutor contended Monday as the most serious case yet went to trial in the Jan. 6, 2021, attack on the U.S. Capitol.

Stewart Rhodes and his band of extremists were prepared to go to war to stop Joe Biden from becoming president, Assistant U.S. Attorney Jeffrey Nestler told jurors. The group celebrated the Capitol attack as a victory in that fight and continued their plot even after Biden’s electoral victory was certified, Nestler alleged.

“Their goal was to stop, by whatever means necessary, the lawful transfer of presidential power, including by taking up arms against the United States government,” the prosecutor said during his opening statement. “They concocted a plan for armed rebellion to shatter a bedrock of American democracy.”

The defendants are the first among hundreds of people arrested in the Capitol riot to stand trial on seditious conspiracy, a rare Civil War-era charge that calls for up to 20 years behind bars. The stakes are high for the Justice Department, which last secured such a conviction at trial nearly 30 years ago, and intends to try two more groups on the charge later this year.

The trial comes as Trump continues to insist, against much evidence, that the 2020 election was stolen from him, and as vocal pushback against the charges filed against those who entered the Capitol continues in some quarters. The broader reaction could show how the American public, as well as the jury, sees the attack, nearly two years later.

Defense attorneys accused prosecutors of cherry-picking comments from messages and videos and said the government has no evidence there ever was any plan to attack the Capitol. Rhodes’ attorney said his client will take the stand and show that the Oath Keepers had merely been preparing for orders they expected from Trump but never came.

“Stewart Rhodes meant no harm to the Capitol that day. Stewart Rhodes did not have any violent intent that day,” Rhodes’ attorney, Phillip Linder, said. “The story the government is trying to tell you today is completely wrong.”

On trial with Rhodes, of Granbury, Texas, are Kelly Meggs, leader of the Florida chapter of the Oath Keepers; Kenneth Harrelson, another Florida Oath Keeper; Thomas Caldwell, a retired U.S. Navy intelligence officer from Virginia, and Jessica Watkins, who led an Ohio militia group. They face several other charges as well.

They are among roughly 900 people who have been charged in the attack, which temporarily halted the certification of Biden’s victory, sent lawmakers running for cover and left dozens of police officers injured.

In the Oath Keepers case, prosecutors will try to prove that their actions were not a spontaneous outpouring of election-fueled rage but part of a detailed, drawn-out plot to stop Biden from entering the White House.

The Oath Keepers “were prepared in November, they were prepared in December and when the opportunity finally presented itself on Jan 6, 2021, they sprang into action,” Nestler said.

Rhodes began plotting to overturn Biden’s victory right after the election, Nestler said. In November 2020, Rhodes sent his followers a step-by-step plan for stopping the transfer of power based on a popular uprising that brought down Yugoslavia’s president two decades earlier.

Around the same time, Rhodes was also communicating in another internet group — which included Trump ally Roger Stone — that was called “FOS,” or “Friends of Stone,” according to testimony. On Nov. 7, 2020, Rhodes said in a message to that group that he was on his way to D.C. for a possible “op” and was available to meet “face-to-face.”

As December approached, Rhodes’ rhetoric became increasingly violent and desperate, Nestler said.

In messages and comments read to the jury, the Oath Keepers repeatedly warned of violence if Biden were to become president. During a December interview, Rhodes called senators “traitors” and warned that the Oath Keepers would have to “overthrow, abort or abolish Congress.” He described Jan. 6 as a “hard constitutional deadline” for stopping the transfer of power.