January 6 The trial of Thomas Webster, a former NYPD officer, begins.

Among the many violent fights that broke out during the riots in the Capitol, this one stood out.

On one side of the barricade was a Washington, D.C. police officer in light armor who stood with his colleagues to protect the building from a crowd of Trump supporters. On the other hand, waving a flagpole, he was confronted not by any attacker, but by a retired member of the New York Police Department.

Prosecutors said that during their brief meeting on the front lines of the riot, former officer Thomas Webster hit his compatriot on duty several times with a flagpole, then squeezed through the barricades and knocked him to the ground. In response to these accusations, Mr. Webster put forward a bold defense: he claimed that the officer had provoked his aggression and that he was simply trying to defend himself.

The trial of Mr. Webster, the sixth case related to the Capitol attack, to go to trial began in Washington Federal District Court on Tuesday. While several defendants accused of the riot said they planned to defend their position in self-defense, Mr. Webster was the first to try this strategy in a jury trial.

Focusing on a former officer accused of assaulting a law enforcement officer, the Webster case will inevitably revolve around one of the twists of fate uncovered on January 6, 2021, when hundreds of rioters stormed the Capitol and detained lawmakers as they were certifying the results of the presidential elections in 2020.

Many in the crowd that day declared themselves supporters of the police in the spirit of “Back the Blue”. But scores of people have been charged with assaulting law enforcement officers – using fists and various weapons, including hockey sticks, baseball bats, metal clubs and bear spray. According to the prosecutor’s office, about 150 officers were injured.

Mr. Webster, a 54-year-old former Marine, was no ordinary officer: he once served in the personal bodyguard of New York Mayor Michael R. Bloomberg. After turning himself in to authorities last year, some of his former colleagues were surprised to learn that he had erupted in violence at the Capitol. One described him as “just one of the guys”.

From the beginning, however, Mr. Webster’s past life as an officer has played a central role in the investigation and prosecution of his case. At a hearing in June to determine his bail, Judge Amit P. Mehta appeared unable to believe that Mr. Webster was in such a predicament.

“Lots of people come to me,” the judge said. “I am puzzled by their behavior. But especially your behavior is hard for me to put in my head. You were a police officer. You better know.”

In a February 2021 FBI interview, Mr. Webster told agents that he traveled to Washington on January 6 to show support for President Donald J. Trump and protest what he believed was a stolen election. He claimed that as he walked with the crowd from Trump’s speech outside the White House to the Capitol, a “crazy” riot outside the building was already in full swing.

Speaking to the FBI, Mr. Webster admitted that he was “the voice” and “frustrated” that day. But he also claimed that one officer in particular, identified by his initials, NR, encouraged him to “jump the barrier” in what Mr. Webster called a “bar-type moment.”

According to Mr. Webster, Officer N.R. struck first.

“I was hit in the face by a freight train,” Mr. Webster told the FBI, calling the blow “a hard blow to the face.”

Prosecutors are expected to offer a different version of events. In the weeks leading up to the trial, they provided evidence that Mr. Webster went to Washington “armed and ready to fight,” with body armor, a map of the area, and military rations similar to those soldiers eat in the field. They also said Mr. Webster brought the gun with him from New York, although they admit he left it in his hotel room.

At trial this week, the jury is likely to hear testimony from the officer who was assaulted, as well as several Capitol police officers who were at the scene that day. Mr. Webster’s lawyer, James Monroe, has indicated in court documents that he intends to call several representative witnesses to testify on behalf of his client.

Long before the trial began, the government asked Judge Mehta to bar Mr. Webster from defending himself, saying the facts did not support his case. In a ruling this month, the judge said he would allow the defense to be presented but would decide after hearing the evidence whether to have the jury consider it.

Mr. Webster is not the first former police officer to stand trial in connection with the riots. On April 11, Thomas Robertson, a former officer from Virginia, was convicted of all six counts, including congressional obstruction of the official vote count.