Lance Cpl. Stephen Tatum, 27, is accused of killing two sisters aged five and 14 who were shot dead at close range as Marines cleared houses in the aftermath of a roadside bombing that killed a comrade on Nov. 19, 2005.

If convicted on two counts of involuntary manslaughter and one count of aggravated assault Tatum could be jailed for up to 18 years.

The deaths were part of a grim civilian toll from the Marines' actions in Haditha, which initially saw four Marines charged with murder and four officers accused of staging a cover-up.

However, since charges against the Marines were first announced in late 2006, prosecutors overseeing the case the Marines' Camp Pendleton base in southern California have struggled to make the allegations stick.

Four of the eight have had charges against them dropped, while charges of murder have been replaced by the lesser offence of manslaughter in the cases of Tatum and his squad leader, Sgt. Frank Wuterich.

The killings in Haditha are the most serious allegations of war crimes levelled at American forces since the 2003 invasion to topple Saddam Hussein.

The Marines said in a press release issued immediately after the violence in Haditha that 15 Iraqis had been killed by the roadside bomb that claimed the life of Lance Cpl. Miguel Terrazas.

But a subsequent investigation by Time magazine showed that most of the dead were killed as Marines swept through three houses near the site of the bombing, prompting a wide-ranging internal investigation.

Tatum was initially charged with murder, but saw those allegations withdrawn and replaced with counts of manslaughter following a preliminary investigation last year which called for him to be exonerated.

Investigating officer Lt. Col. Paul Ware recommended that all charges against Tatum be dropped on the grounds that he had shot at the children because another Marine, Wuterich, had started firing.

"In the mere seconds Lance Corporal Tatum had to make a decision, he acted in accord with training, to engage targets that a fellow Marine was firing at, without time to fully assess the situation and reflect on what Sergeant Wuterich was doing," Ware wrote.

"I believe Tatum's real life experience and training on how to clear a room took over and his body instinctively began firing while his head tried to grasp at what and why he was firing," Ware added.

"By the time he could recognize that he was shooting at children, his body had already acted."

Former Marine judge advocate Gary Solis, an expert in the Laws of War at Georgetown University, said that Tatum's trial could hinge on what the Marine knew before he entered the room where the children were shot.

At an earlier pre-trial hearing in Wuterich's case, Lance Cpl. Humberto Mendoza said Tatum had told him to shoot the people inside the room even after being made aware they were women and children.

"Rather than the rules of engagement, I think the case is going to turn on what (Tatum) knew going into the house," Solis told AFP, describing the cases being prosecuted as "highly unusual."

"Whenever there is doubt over whether a shooter had done the right thing, the benefit of the doubt is always given to the shooter," Solis said.

"The convention has always been that you don't charge a guy for his actions taken in the heat of the moment.

"This is the first case I know of where U.S. forces have been charged for judgements made while under fire. That is highly unsual."

Prosecutors would also face a massive challenge in attempting to build a case years after the events in question, with no forensic evidence and witness testimony that often contradicted itself, Solis said.

"The problems are going to be exactly as they would be in a civilian trial. Time always favors the defense. Memories fade, inconsistencies arise.

"In this case, real evidence is absent. The main problem is that memories fade, grow foggy and that always benefits the defense."

Jury selection and pre-trial motions in Tatum's court martial are expected to dominate on March 27 and March 28, with opening statements and witness testimony expected to get underway next week.

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