RAMSTEIN AIR BASE, Germany — A Ramstein airman will serve five months in jail, but can remain in the Air Force, after being convicted at a court-martial Friday of involuntary manslaughter in the death of his best friend.

Airman 1st Class Juston Besaw never disputed that he threw the punch that led to Airman 1st Class Zackery Allen’s death.

The blow, medical experts on both sides agreed, caused Allen’s left vertebral artery to lacerate, which led to severe bleeding inside his skull and brain death. Three days later, he was removed from life support at Landstuhl Regional Medical Center. He was 20.

In addition to the five-month jail term, Besaw was reduced in rank to E-2. He was facing a maximum 10 years in federal prison, a possible dishonorable discharge and loss of all pay and allowances.

Nearly two days of the weeklong trial were devoted to medical testimony describing how that single punch, which left no visible bruising and no fractured bones, could be deadly. The mechanics and force of the blow were such that it hyper-extended and rotated Allen’s neck just enough to stretch and tear the artery, likely an “uppercut” to the left side of the jaw, according to Dr. (Cmdr.) Timothy Monaghan, a forensic pathologist and Armed Forces Institute of Pathology deputy medical examiner, who testified for the prosecution.

The defense’s medical expert had his own theory: That Allen was months away from dying because of an advanced aneurysm that ruptured when Besaw hit him.

“If it was a normal healthy vessel, he would have ended up with a slap to the face,” said Dr. Harry Bonnell, a forensic pathologist from San Diego, Calif. His conclusions were based on his examination of microscopic slides of Allen’s torn artery.

The deadly incident occurred following a night of partying at a Ramstein village apartment to celebrate Besaw’s recent engagement. Allen was to be his best man.

As the evening wound down, Besaw and Allen lay on a couch, and Besaw, who was asleep, passed gas in Allen’s face, according to testimony from Scott McCormack, a friend of the two men.

Allen sat up and threw beer on Besaw. Besaw countered with a slap to the back of Allen’s head, McCormack testified. The two started wrestling, “play-fighting” as one witness called it, creating such a ruckus that they were asked to leave.

Besaw took the witness stand on the third day of the trial. He testified that once outside, he tried to find Allen to ask him why he poured beer on him.

Allen came toward him and “swung at me, on the left side of my face,” he told the court. “I hit him back in the left temple,” he said. “I wasn’t trying to hurt him.”

“Did you think that a single punch would end up causing the death of your best friend?” Maj. Charlton Meginley, Ramstein senior defense counsel, asked Besaw.

“Absolutely not,” Besaw answered.

The government’s attorneys argued that Besaw had never claimed self-defense in any discussions until the court-martial.

“Given one last time to honor his fallen friend ... he pointed his finger” at Allen and called him the aggressor, said lead prosecutor Capt. Etienne Miszczak.

After a sentencing hearing late Friday, Besaw, 26, tearfully apologized to Allen’s grieving mother, Vicki.

Vicki Allen said after the trial that she forgave Besaw but was shocked that he received only five months in jail.

“I don’t think the punishment fits the crime,” she said, adding that her son “was everything to me.”