Military jurors will hear opening arguments in the trial of Staff Sgt. Frank Wuterich. In Nov. 2005, he led a U.S. Marine squad that killed 24 Iraqi civilians in Haditha, including unarmed women and children.
The civilians were killed during a house-clearing operation, following a roadside bomb explosion that killed one Marine.
Prosecutors argue that Wuterich's actions were criminal. Wuterich maintains he thought he was operating within military combat rules.
The incident is considered one of the war's defining moments- it came on the heels of the release of photos of prisoner abuse by U.S. soldiers at Abu Ghraib prison, further tainting America's reputation.
Eight Marines were initially charged in the Haditha killings. Charges were dropped or dismissed against six, and one was acquitted.
Wuterich has completed his service but can't leave the military until his case has been resolved. He has been working a desk job at Camp Pendleton awaiting trial.
The Associated Press contributed to this story.