WASHINGTON, D.C. (AP) — The founder of the extremist group Oath Keepers and four of his associates planned an “armed insurrection” to keep President Donald Trump in power, federal prosecutors said Monday in what was the most serious case yet handled in January . March 6, 2021 attack on the US Capitol.

Stuart Rhodes and his group of extremists were prepared to go to war to prevent Joe Biden from becoming president, Assistant US Attorney Jeffrey Nestler told jurors. The group celebrated the attack on the Capitol as a victory in that struggle and continued its plot even after Biden’s election victory was certified, Nestler argued.

“Their goal was to stop the lawful transfer of presidential power by any means necessary, including by taking up arms against the United States government,” the prosecutor said during his opening statement. “They devised a plan of armed insurrection to destroy the foundation of American democracy.”

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

The trial comes as Trump continues to insist, in the face of mounting evidence, that the 2020 election was stolen from him, and as vocal protests continue in some quarters against the charges against those who entered the Capitol. The broader response may reveal how the American public, as well as juries, view the attack nearly two years later.

The lawyers accused prosecutors of cherry-picking comments from the messages and videos and said the government has no evidence that there was ever a plan to attack the Capitol. Rhodes’ attorney said his client would come forward and show that the oath keepers were simply preparing for orders they expected from Trump, but never came.

“Stuart Rhodes didn’t have the Capitol in mind that day. Stuart Rhodes had no violent intentions that day,” said Rhodes’ attorney Philip Linder. “The story the government is trying to tell you today is completely wrong.”

On trial with Rhodes, of Granbury, Texas, are Kelly Maggs, leader of the Florida chapter of Oath Keepers; Kenneth Harrelson, another Florida Oath Keeper; Thomas Caldwell, a retired U.S. Navy intelligence officer from Virginia, and Jessica Watkins, who led the Ohio militia group. A number of other charges have also been brought against him.

They are among about 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 outburst of election-fueled anger, but part of an elaborate, long-running conspiracy to keep Biden out of 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 went into action,” Nestler said.

Rhodes began plotting to overturn Biden’s victory immediately after the election, Nestler said. In November 2020, Rhodes sent his followers a step-by-step plan to stop the transfer of power based on the popular uprising that ousted the president of Yugoslavia two decades ago.

In the days between the riots and Biden’s inauguration, Rhodes spent more than $17,000 on firearms parts, ammunition and other items, prosecutors say. Shortly after the uprising, Rhodes was secretly recorded saying his “only regret was that they should have brought rifles,” Nestler said.

Among those who could testify in the weeks-long trial are three jurors who pleaded guilty to conspiracy and are cooperating with prosecutors in hopes of getting lighter sentences. Among them is a man who says that after arriving in Washington, Maggs told him that another Florida oath keeper had brought explosives in his van.

Caldwell’s attorney said his client is a disabled veteran who didn’t even know about Oath Keepers until November 2020. Attorney David Fisher called Jan. 6 a “black eye” for the country, but said Caldwell had simply come to Washington “to see his wife” and had not even planned to go to the Capitol before Trump’s speech at the Ellipse before the riots.

“Mr. Caldwell couldn’t get out of the paper bag,” Fisher said. “I came here to clear his name.”