Pte John Jodka is one of eight implicated in an attack
A US marine has received an 18-month prison sentence for his part in killing an unarmed man in Iraq.
Pte John Jodka is one of eight servicemen implicated in the attack seven months ago in Hamdaniya, in which a 52-year-old man died.
The judge said he would have sentenced Jodka to five years, but he was bound by a "very fortuitous" pre-trial deal.
Jodka, who apologised to the victim's family, is expected to give evidence against the other defendants.
As part of a plea bargain, Jodka pleaded guilty in October to charges of assault and conspiracy to obstruct justice, while prosecutors dropped other charges including murder and kidnapping.
The BBC's David Willis, in California, says other more senior officers among the defendants are expected to face kidnapping and murder charges.
They could face the death penalty if convicted in trials that are likely to drag on towards the end of 2007, our correspondent says.
By giving evidence against them, Jodka stands to receive a general discharge from the marines, although the judge, Lt Col David Jones, said that he would have given him a dishonourable discharge.
"You have a very fortuitous pre-trial agreement," the judge told Jodka.
At 20 years of age, Pte Jodka is the youngest and lowest-ranking member of a group of marines who were deployed in Iraq earlier this year.
It is alleged that the eight-strong team went looking for a suspected insurgent.
When they failed to find him the men became frustrated and dragged a disabled grandfather from his home, bound and beat him and then shot dead.
In court, Pte Jodka apologised to the family of the Iraqi man who died and to fellow members of the Marine corps.
He also talked about conditions in Iraq, referring to the fear and frustration he and his colleagues felt there and telling the court that he often felt poorly trained for the duties he had been asked to undertake, including counter-terrorism.
The case comes on the heels of the US mid-term elections, where Iraq featured as a key issue for voters.
Our correspondent says the case will feed into public disquiet about the conduct of the war in Iraq.
In another high profile case, soldiers from the same base as Jodka - Camp Pendleton - are facing the prospect of prosecution over the deaths of 24 Iraqis at Haditha in November 2005.