John Hasenbein, who has escorted Donald J. Trump’s pick for defense secretary to meetings on Capitol Hill, said he was unjustly prosecuted for the 2019 episode.

When Pete Hegseth visited senators on Capitol Hill this month in an effort to show that he has the qualifications and judgment to lead the Defense Department, he was escorted by a security guard with a dark episode in his past.

The guard, a former Army Special Forces master sergeant named John Jacob Hasenbein, left the military after a 2019 training event in which witnesses said he beat a civilian role player — kicking him, punching him and leaving him hogtied in a pool of his own blood.

Mr. Hegseth’s choice of Mr. Hasenbein as a security escort is the latest instance in which he has stood by soldiers accused of crimes. He has repeatedly criticized military leaders as being too “woke” and waging a “war on warriors.”

In this case, Timothy Parlatore, Mr. Hegseth’s lawyer, said Mr. Hasenbein was the “victim of unjust treatment by a broken military justice system,” and Mr. Hegseth was proud to work with him. He added, “It is stories like his that demonstrate the change needed in the Department of Defense.”

The Army charged Mr. Hasenbein with aggravated assault and reckless endangerment. A military jury found him guilty of the assault charge in a court-martial in 2020, according to Army records. But the judge overseeing the case declared a mistrial after learning that a friend of Mr. Hasenbein had been talking to a juror throughout the trial, court records show. The Army did not retry the case.

“This guy has real anger issues,” Ahmed Altameemi, the man whom Mr. Hasenbein was charged with assaulting, said in an interview. “He is not a safe man. I don’t know why they gave him this position. Don’t they do any background checks?”

Mr. Hasenbein provided The New York Times with several official records involving his service and the case against him, along with a statement in a text message: “I have no conviction and I was honorably retired after 22 years of service. That’s all you need to know.”

Timothy Turner, senior counsel at the firm that represented Mr. Hasenbein in a defamation case that he filed after the court-martial proceedings, said that Mr. Hasenbein agreed to retire rather than face another trial. Mr. Hasenbein claimed in that case that he was effectively forced out of the military because of false and misleading statements by employees of the independent contractor who conducted the exercise.

It is unclear whether Mr. Hegseth, President-elect Donald J. Trump’s pick for one of the most important cabinet posts, knew of Mr. Hasenbein’s record when he hired him. But some details of his case have been online for years.

Michèle A. Flournoy, an under secretary of defense during President Barack Obama’s first term, said having Mr. Hasenbein as security “is not a good look” for Mr. Hegseth. She said Mr. Hegseth had seemed to display “a pattern of disregard for the military institutional efforts to hold its own accountable.”

Mr. Hegseth’s path to confirmation as defense secretary has been far from smooth.

He has faced allegations of excessive alcohol use. He has said that he has never had a drinking problem but that he would not drink at all if he becomes defense secretary. In 2017, he was accused of raping a woman, an allegation that he denied and that resulted in no criminal charges but led to an out-of-court financial settlement with the woman. At first some Republican senators appeared to have reservations about Mr. Hegseth, but Mr. Trump has stood by him.

The exercise that preceded the end of Mr. Hasenbein’s military career was held at Fort Knox, Ky., for his Fifth Special Forces Group unit before it deployed overseas. It was organized with a private contractor, F3EA, to simulate a hostage rescue from a terrorist cell. Mr. Altameemi, now 38, was employed by the company to play the role of an ISIS fighter with information about a hostage whom Mr. Hasenbein’s team was supposed to free.

Mr. Hasenbein, now 45, had deployed to Iraq eight times and was awarded two Bronze Stars, and was a senior soldier in a group that detained Mr. Altameemi. The soldiers forced Mr. Altameemi to the ground and held him down, according to a video reviewed by The Times.

Mr. Altameemi, a former member of the Iraqi Army’s elite counterterrorism service who is now an American citizen living in Texas, said he had been a role player in other military exercises and never had any issues, but this one turned inexplicably brutal.

“They were supposed to ask me where the hostage is at,” he said in an interview. “But they didn’t give me a chance. They just started hurting me bad.”

In a memo recounting the episode, a supervisor with F3EA said Mr. Hasenbein rubbed Mr. Altameemi’s face into a concrete pillar, kicked him and delivered repeated “hammer-fist” strikes to his ribs and “knee strikes” to his face.

Another company employee involved in the exercise said, “The other guys realized things were wrong and started to calm down.” But Mr. Hasenbein “didn’t calm down — he never calmed down,” he said.

A third said he found Mr. Altameemi face down and hogtied, moaning in pain, with a pool of blood around his head.

Among other injuries, Mr. Altameemi was left with a broken nose, a broken tooth, a sprained shoulder, a scalp hematoma and blunt facial trauma, according to memos and statements by company employees about the episode. He was taken to the hospital after the exercise.

Mr. Altameemi said Mr. Hasenbein knew that a training exercise was not supposed to happen like that. “He is a man with experience, a master sergeant — he knows this is training on American soil,” Mr. Altameemi said.

During the court-martial that followed, an expert for the defense said that based on Mr. Altameemi’s behavior during the exercise, Mr. Hasenbein’s actions were reasonable. The defense also asserted that a commander had pressured two soldiers to change their statements that nothing wrong had happened during the episode.

The prosecutor’s expert testified that Mr. Hasenbein’s use of “knee strikes” against Mr. Altameemi, who lacked protected gear, amounted to excessive force.

The military jury found Mr. Hasenbein guilty of the aggravated assault charge but acquitted him on a charge of reckless endangerment. He was sentenced to no punishment.

After the court-martial proceeding, the judge discovered that one of Mr. Hasenbein’s fellow Special Forces soldiers, a friend, had repeatedly talked to a juror. Among other advice, he told the juror that physical aggression was “pretty normal” in training scenarios, according to the judge’s decision to declare a mistrial.

In his defamation case against F3EA, filed in December 2020, Mr. Hasenbein accused the firm’s employees of a conspiracy to harm his reputation that effectively forced him out of the military. Mr. Parlatore, Mr. Hegseth’s lawyer, said on Sunday that Mr. Hasenbein voluntarily retired because he would not have been promoted after the criminal case.

The defamation case was dropped in September 2021 with the agreement of both sides.

After leaving the military, Mr. Hasenbein worked for several personal security companies, according to his LinkedIn account and a former employer.