Eight US marines have been charged over the deaths of 24 Iraqi civilians in the town of Haditha last year.
Four are accused of unpremeditated murder and four others are charged with attempting to cover up the incident.
Men, women and children were killed by marines who said they were under attack from insurgents at the time.
Meanwhile, US Secretary of State Condoleezza Rice has said that despite the cost in lives and money, the war in Iraq has been "worth the investment".
She told the Associated Press news agency that this was because once Iraq emerged as a stabilising factor it would produce a very different kind of Middle East.
The BBC Washington correspondent says the comments show that, despite the calls of many advising the White House to treat the Arab-Israeli conflict as the real key to peace, Ms Rice still clearly believes that Iraq should be the centrepiece of US Middle East policy.
Leadership debate
Squad leader Staff Sgt Frank Wuterich has been charged with the unpremeditated murder of 12 Iraqis and ordering his troops to kill six people, his lawyer Neal Puckett said.
Lance Cpl Justin Sharratt has also been charged with three such counts, his legal representatives say.
It is the biggest US criminal case to emerge from the war in Iraq in terms of Iraqis killed.
HADITHA CHARGES
The US military initially said the civilians died in unrest. If found guilty of second-degree murder, the marines could face life imprisonment.
The BBC's Nick Miles in Washington says the case has provoked debate about the training and leadership of American troops in Iraq and the way in which counter-insurgency operations are being conducted there.
The defence team say a group of marines from Kilo Company in America's First Marine Division were engaged in a furious battle on 19 November 2005 in Haditha after a roadside bomb exploded, killing one marine and injuring two others.
It is known that five unarmed men were shot dead in a car when they approached the scene in a taxi and others, including women and children, died in three houses over the next few hours.
Those who died included a 76-year-old man and a three-year-old child. There were also several women among the dead.
An initial marine press statement said that some civilians were killed in the initial explosion and others in crossfire by insurgents.
But local people say that there were no bullets fired other than by the marines.
The defence lawyers accept that innocent civilians may have died during the chaos but they deny premeditated killing.
Iraqi Prime Minister Nouri Maliki has called the deaths a "terrible crime".
Dangerous place
According to the charge sheets, Sgt Wuterich ordered his men to "shoot first and ask questions later" when they entered a house, the Associated Press news agency reported.
There was no full US investigation into what happened until three months later when video footage that was taken by a local human rights activist of the aftermath reached Time Magazine.
Once their report showed flaws in the initial marine statement, an investigation began.
The Haditha inquiry is just one of a number the US military has been conducting into incidents of alleged unlawful killings by US forces in Iraq.
Haditha, 96km (60 miles) north of Baghdad, is the furthest in a string of settlements up the Euphrates that have been prominent in the Sunni insurgency.
Anbar province, which includes Haditha, remains the most dangerous place in Iraq for Americans.
About 100 US troops have been killed there since August this year.
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