US soldiers killed 504 innocent Vietnamese villagers at My Lai in Vietnam on March 16th 1968.
Women and children were raped, cattle were slaughtered, houses burned and crops destroyed.
It became known as the 'My Lai massacre' and its significance still resonates today.
Only one solider, Lt William Calley, was prosecuted for what happened on that day.
The My Lai Tapes - Part One
In the first episode of this two part series, Robert Hodierne, an American military journalist pieces together the horror and barbarism that swept through the village of My Lai 40 years ago.
He reconstructs the events that led up to that day with exclusive archive recordings from the US Army's own investigation into the massacre.
He also speaks to surviving victims and to some of the perpetrators of that historic day.
Massacre at 'Pinkville'
The Vietnam war was America's attempt to stop Communists from toppling one country after another in South East Asia.
The US fought there from the early 1960s until 1975. It became America's longest war.
By 1968 there were nearly half a million US troops in Vietnam and more were being hurriedly drafted in to fight.
Most of the men sent into battle in My Lai were very inexperienced and very few of them had seen combat.
It was an unconventional war where the enemy wasn't clear - everybody and everything that moved was considered a target.
My Lai was a search and destroy mission - everything from housing and food to people was to be wiped out.
The operation lasted four hours and in that time, US troops had sustained one death and eight injuries.
In contrast 504 villagers were murdered and, in some cases, raped and mutilated.
The operation was touted as a success by the US military and the truth of what really happened remained a secret for a whole year.
The real truth
As misgivings about the Vietnam war grew, news of what happened at My Lai gradually began to surface.
The US Army then ordered its own investigation into the massacre, it was called 'The Peers Inquiry'.
In the second part of the My Lai Tapes, you can hear, for the first time, the recordings of the US Army's internal inquiry into the massacre.
The tapes lay forgotten for nearly 40 years until they were tracked down by the British journalist Celina Dunlop, who spent two years trawling through the national archive near Washington.
The Peers Inquiry proved that US soldiers raped and killed hundreds of civilians.
It showed just how badly trained and ignorant many of the young men were about the proper treatment of civilians during war.
The conclusions of the inquiry went on to make important and lasting recommendations about how soldiers should be trained in the laws of war.
First broadcast 23rd April 2008.
More from this series