The My Lai massacre, which took place on the morning of March 16, 1968, was a watershed in the history of modern American combat, and a turning point in the public perception of the Vietnam War.

Paul Meadlo, father of two, and one of the soldiers involved in the massacre

In the course of three hours more than 500 Vietnamese civilians were killed in cold blood at the hands of US troops. The soldiers had been on a "search and destroy" mission to root out communist fighters in what was fertile Viet Cong territory.

Yet there had been no firefight with the enemy - not a single shot was fired at the soldiers of Charlie Company, a unit of the Americal Division's 11th Infantry Brigade.

The 48th Viet Cong Battalion - the intended target of the mission - was nowhere to be seen.

When the story of My Lai was exposed, more than a year later, it tarnished the name of the US army. Most Americans did not want to believe that their revered GI Joe could be a wanton murderer.

My Lai was the sort of atrocity American patriots preferred to associate with the Nazis.

Many of those killed at My Lai were women

Charlie Company

Charlie Company had arrived in Vietnam three months before the My Lai massacre.

By then the US - fighting alongside the South Vietnamese army - was deeply entrenched in war against North Vietnam's communist forces. The United States's had deployed nearly 500,000 soldiers in Vietnam, a commitment which cost it $2 bn every month.

In January 1968 the Viet Cong guerrillas and the regular North Vietnamese Army launched a joint attack on US positions, known as the Tet Offensive. Washington maintained it could win the war, but on the ground morale among its troops was low.

Charlie Company was down to 105 men by mid-March of that year. It had suffered 28 casualties, including five dead. Some of its soldiers had already begun to drift towards brutal tactics for which they appeared to enjoy impunity.

A military map used in the My Lai operation

My Lai

The brief for its March 16 mission was to prise out the Viet Cong, whose elusive troops were thought to be hiding in My Lai - a hamlet of the Son My village.

Two platoons moved in shortly after 8pm in the morning, while a third held back for "mopping up" duties. Both platoons soon splintered and once the shooting started it seemed to spark a chain reaction.

Soldiers went berserk, gunning down unarmed men, women, children and babies. Families which huddled together for safety in huts or bunkers were shown no mercy. Those who emerged with hands held high were murdered.

Some lucky villagers, like these two children, survived the massacre

Some of the 120 or so soldiers opted out of the killing spree, but troop commander Lt William Calley was not one of them. In one incident, Lt Calley ordered two of his men to fire on a group of 60 civilians they had rounded up. When one refused, Calley took over and, standing 10 feet from the crowd, blazed his gun at them.

Elsewhere in the village, other atrocities were in progress. Women were gang raped; Vietnamese who had bowed to greet the Americans were beaten with fists and tortured, clubbed with rifle butts and stabbed with bayonets. Some victims were mutilated with the signature "C Company" carved into the chest.

By late morning word had got back to higher authorities and a cease-fire was ordered. My Lai was in a state of carnage. Bodies were strewn through the village. The death toll totalled 504.

Only one American was injured - a GI who had shot himself in the foot while clearing his pistol.

• Part 1 - Murder in the name of war: My Lai
• Part 2 - My Lai: the cover-up
• Part 3 - My Lai: the whitewash
• Part 4 - Timeline: Vietnam war 1945-1975
• Part 5 - Heroes of My Lai honoured