"A U.S. Marine and 15 Iraqi civilians were killed yesterday from the blast of a roadside bomb in Haditha," read a U.S. military press release in November 2005. Four months later, Time magazine would report that it was U.S. Marines -- not a roadside bomb -- who were responsible for the deaths of unarmed Iraqi civilians. Soon after, Rep. John P. Murtha (D-Pa.) would claim the Marines killed the Iraqis "in cold blood," igniting a media firestorm which labeled Haditha a "massacre" and one of the worst atrocities of the Iraq war. But what really happened that day reveals a far more complex story that gets to the heart of the war troops are fighting.

Through interviews with the highest levels of the U.S. military, personal accounts from Marines involved, documents obtained by FRONTLINE, unmanned drone footage of the actual day's events, and an exclusive television interview with an intelligence officer who watched the day unfold, FRONTLINE investigates what occurred in Haditha, how the rules of war are interpreted in theory and in battle, and what the events of Haditha and their aftermath say about the war in Iraq.

The story begins in October 2005, when the Marines of Kilo Company of the 3rd Battalion, 1st Marine Regiment arrived in Haditha, a town in the Sunni-dominated Anbar province. They anticipated heavy combat similar to what the company had experienced in November 2004, during the second battle of Fallujah, where civilians were evacuated, allowing the Marines to aggressively pursue insurgents house to house in the most intense urban combat since Vietnam. Kilo Company was the tip of the battalion's spear in that battle, leading the charge and emerging as heroes. Now the company found itself in a much more complex situation, in a relatively quiet town where insurgents often hid among civilians.

But all that would change on Nov. 19, 2005, when a Humvee in a convoy carrying 12 Marines was hit by an IED (improvised explosive device), instantly killing Lance Cpl. Miguel Terrazas. At first the Marines said the explosion also killed 15 Iraqis. The incident was barely covered by the press until Time magazine received a gruesome video from an Iraqi human rights group showing the dead Iraqis, including women and children, maimed by gunshot wounds, not by an IED, as the original press release had stated. The total number of civilians reportedly killed that morning rose to 24 when a number of "insurgents" shot by the Marines were reclassified as "noncombatants."

Some of the Iraqi civilians were killed inside their homes. A young girl who survived the Haditha incident told FRONTLINE what she says she saw in the first house that was raided: "They came in and killed my father. Then they killed my grandfather," said Iman Walid, who was 9 years old at the time. "After that, they threw a grenade in the hall where we were sitting. Everyone died except me and my brother."

In December 2006 the Marine Corps filed charges against eight Marines, resulting in one of the most significant criminal cases against U.S. troops during the Iraq war. The charges ranged from unpremeditated murder against four enlisted men involved in the shootings to dereliction of duty against four officers for failing to investigate the killings. In extensive pretrial hearings, the Marines maintained that they were following proper rules of engagement (ROE) and that they had done nothing wrong either during the incident or afterward. In a military courtroom, Lance Cpl. Stephen B. Tatum recalled that his squad leader had instructed him and other Marines to treat the first house they entered as hostile, and that he heard someone racking an AK-47 gun before tossing in a grenade. "I didn't know there was women and children in that house," Tatum, who is awaiting court-martial, said. "Otherwise, I would have physically stopped everybody from going back there to shoot anybody."

But what transpired in the hearings has been surprising and has revealed the complex story of what happened that day. Of the eight Marines originally charged, four have since had their charges dropped entirely. While the other four still face courts-martial, murder charges have been reduced to manslaughter for those men involved in the shootings. Staff Sgt. Frank D. Wuterich recalled details from that day in a hearing before a military judge: "I advised the team something like, 'Shoot first and ask questions later,' or 'Don't hesitate to shoot.' I can't remember my exact words, but I wanted them to understand that hesitation to shoot would only result in the four of us being killed." Wuterich, whose court-martial is currently set to begin on March 3, 2008, said he and the other Marines in his squad were just doing what they were trained to do. But many Iraqis remain angered and feel justice has yet to be served. Iraq's deputy prime minister, Dr. Barham Salih, told FRONTLINE: "These crimes are heinous crimes, terrible acts. I would even call what happened in Haditha a war crime."

Marine Corps commandant Gen. James T. Conway told FRONTLINE that Haditha and other cases were turning points and "caused us to take a look at how we are conducting our training, how we are emphasizing ROE, how we prepare a battalion to go into combat," he said. "In that moment of crisis, it may be 120 degrees, rounds may be impacting, your buddy may be bleeding, but you've got to do the right thing."

Gary Myers, the defense attorney for one of the accused Marines, told FRONTLINE, "Haditha will be the case that causes the military to come to grips with the rules for insurgency combat in a way they never had to before."

To understand the impact of the Haditha incident, FRONTLINE embedded with Kilo Company in the summer of 2007. At the same time these Marines are being asked to carry out military missions, they are also supposed to gain the trust of Iraqi civilians while they work in the shadow of Haditha. Some worry that these dual objectives may cause troops to hesitate on the battlefield for fear of facing legal jeopardy. "The lines are certainly blurred in an environment like this," 1st Lt. Alex Martin told FRONTLINE. "You know the training is there so that it becomes reactive, and they respond when they need to respond. But at the end of the day it's this human element, where the junior subordinate leader might be a 19-, 20-year-old corporal who has to make a decision. … He is judge and juror in that split second in that environment. And that's the moral authority that these young men have."

Editor's Note: Read an update on the Marines' court-martials.