A US marine has been jailed for an unprovoked, unexplained attack on a 60-year-old woman left traumatised by the barrage of punches to her head.
Lance Corporal Taylor Wyatt Elwood, 20, was drunk and can't remember punching and throwing the woman against a guard rail and cars at Brisbane's Enoggera barracks car park about 3.30pm on July 3.
As he attacked after dragging her from her car, she screamed for help as loudly as she could.
He told her "just be quiet" in a tone she described as having "no emotion".
The woman, a barracks staffer, said she felt she had no control of her body but "willed herself not to black out".
"(He seemed) obsessed with her head and kept banging it and punching it over and over," police prosecutor Glenn Whittle said.
"She felt like a rag doll being thrown around. She believed she was going to die."
When two men stopped after seeing Elwood standing over the bloodied victim, he grabbed, punched and bit one of them who was a federal police officer.
The woman escaped with bruises and lacerations for which she was hospitalised.
Elwood will spend three months in jail after pleading guilty to two counts of assault.
He will be dishonourably discharged and possibly court-martialled by the US Marine Corp upon release, while the woman has been so psychologically impacted she has not been able to return to work.
After the attack, he wrote to the woman to say sorry but Magistrate Tina Previtera noted the victim deemed it a "hollow" apology since it did not say why he had done it.
"It was so violent it's a surprise there aren't more physical injuries, Ms Previtera said.
"There are, as a result of the incident, indentations to her own vehicle where her head was slammed into it.
"You're a young, able-bodied, strong US marine and given the complainant is a 60-year-old woman who you have never met, she had never met you, the violence visited upon her was frightening."
Defence solicitor Dave Garratt said Elwood could not explain his actions but speculated someone in a car resembling the victim's had yelled out to him earlier.
Mr Garratt said Elwood, from New Mexico, had a high sensitivity to alcohol given he was not of legal drinking age in the US.
The promising football player, who chose the marines over a college scholarship, had seen his career go "up in flames" over the incident, Mr Garratt said.
"He's not only brought shame to himself but the US Marine Corp and also to his country," Mr Garratt said.
Magistrate Previtera labelled his offending "completely out of character" given his "impeccable record of service" and accepted his remorse.
She handed him a 15-month jail sentence to be suspended after three months.
He will also pay $3000 restitution to the woman.
His unit will depart for the US without him on Sunday.