The sentence was handed down Friday night after nearly five hours of testimony. Sgt. Joseph Leahy, 28, will also have his rank reduced to private, his pay forfeited and be dishonorably discharged.
Leahy was found guilty on charges of murder and conspiracy to commit murder by the nine-person jury who had heard testimony about the killings at the court-martial at the Army's Rose Barracks Courthouse since Wednesday.
Leahy was acquitted of murder in a separate incident involving the death of another Iraqi in January 2007. Wearing his dress uniform, he sat impassively as the verdicts were handed down by the foreman of the jury made up of officers and enlisted personnel.
Leahy pleaded not guilty to charges of premeditated murder, conspiracy to commit premeditated murder and obstruction of justice in the deaths of a total of four Iraqi prisoners who were dumped in a Baghdad canal in 2007 after they were killed.
He had faced charges, including murder, for the separate incident in January 2007.
Leahy, of Lockport, Illinois, confessed to military investigators that he shot one of the prisoners point-blank in the back of the head with a 9mm pistol.
In closing arguments earlier, Leahy's civilian lawyer, Frank Spinner, argued that Leahy went along with the killings because he was dazed from a lack of sleep and numb from being in a war zone for months. It was a sentiment bolstered on Thursday in testimony from Col. Charles Hoge, a doctor and director of psychology and neuroscience at the Army's Walter Reed Army Institute of Research.
He testified that Leahy was unable to reason properly because of the constant danger of living and operating in a war zone and getting little sleep for months on end.
"The tragedy resulted not so much by design but rather the working of fear, danger and madness attendant on many combat operations," Spinner said in his closing arguments.
The Iraqi prisoners were taken to the U.S. unit's operating base in Baghdad for questioning and processing, although there wasn't enough evidence to hold them for attacking the unit. Later that night patrol members took the Iraqis to a remote area and shot them in retribution for the attacks against the unit, according to testimony.
Leahy, Master Sgt. John Hatley, 40, and Sgt. 1st Class Joseph Mayo, 27, are accused of pulling the trigger, the jury of seven men and two women was told.
But prosecutors contended that Leahy knew what he was doing after the four Iraqis had been taken into custody following a shootout with a patrol that included five other accused soldiers.
"The defense can't just stand there and throw their arms up and say: 'We were protecting ourselves from future harm,"' Army Capt. Derrick Grace, the lead prosecutor, said, adding that the killings were the result of a breakdown of discipline and moral responsibility.
All of the accused were with the 1st Battalion, 18th Infantry Regiment, 2nd Brigade of the 1st Infantry Division in Iraq, which is now part of the Germany-based 172nd Infantry Brigade.
Three soldiers are scheduled for later courts-martial. Sgt. Charles Quigley, 28, of Providence, Rhode Island, faces one charge of conspiracy to commit premeditated murder. Mayo and Hatley are charged with premeditated murder, conspiracy to commit premeditated murder, and obstruction of justice.