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In 2004, Marines fought what may have been the most intense urban warfare since the Vietnam War. They pushed Iraqi insurgents out of Fallujah. It was house-to-house and sometimes hand-to-hand combat.
Nelson, who was then a corporal, from Camp Pendleton, was back for his second combat tour. He also was one of the Marines who spearheaded the Iraq invasion. Nelson was in Fallujah from day one.
"It was very intense, it was very intense," said Nelson. "I lost my best friend, Juan Segura, to a sniper shot."
Now a sergeant, Nelson has had to face murder charges from what happened later that day, along with two of his fellow Marines. The Marine Corps filed charges that prompted a nationwide petition campaign on their behalf.
"Rounds coming from everywhere but someone visually seen movement from that particular house," said Nelson. "It was a pretty big house."
Nelson says inside the house members of the squad spread out to search. They found four apparently unarmed men inside the house, along with many rounds, spent shell casings and hidden weapons with warm barrels. Nelson and his comrades never denied killing the four men.
Later Weemer was interviewing with the Secret Service and in response to a question, mentioned the shootings. Three years after the incident, the Marine Corps filed murder charges.
Retired Marine colonel and Iraq combat veteran G.I. Wilson was one who took up their cause.
"You notice in this case there were no officers involved one way or the other, no officer speaking out for theses Marines," said Wilson.
Weemer and Nazario were tried, Weemer by a panel of Marine veterans. Nazario, because he was discharged, was tried by a civilian jury.
Former Marine William McNulty heard about the case. It was the first time a civilian jury had tried a former serviceman for actions in combat.
"What's at stake is second-guessing the decisions of Marines in combat from the comfort of our own home, really, and I find that very unfair," said McNulty.
The civilian jury agreed that they shouldn't be second-guessing Marines in combat. Nazario was acquitted. The Marine panel acquitted Sgt Weemer.
Yet the Marine Corps continued the case against Nelson. Nelson and his comrades went to jail, rather than testify against each other. Marines and former Marines started a nationwide campaign.
There was a nationwide petition drive and countless e-mails to put pressure on the Marine Corps.
"I think they were made aware of it," said Joseph Low, Nelson's attorney. "The Marine Corps wants to be sensitive to public opinion. They do care."
Nelson's attorney, a former Marine, says the three followed orders in Fallujah.
"There are facts that confirm through radio messages, orders, that the four jihadists should be killed," said Low.
Not bitter, Nelson still loves his Marines.
"They gave me a second chance at life," said Nelson. "They really taught me how to become a man. I really appreciate that and I owe them my life."
Nelson intends to re-enlist. But an expert testified Tuesday afternoon that Nelson is suffering from a severe case of post-traumatic stress disorder. There was a parade of Marines from captain to sergeant major testifying they would feel comfortable going to war with Nelson. Nelson says he wants to go to Afghanistan.