Forty years on, and “My Lai” is synonymous with “massacre”.

The killing of Iraqi civilians at Haditha has often been referred to as a modern-day My Lai.

The name is shorthand for slaughter of the defenceless, the benchmark of American wartime atrocity.

The murders of 504 men, women, children and babies happened in a northerly province of South Vietnam on 16 March 1968.

It proved to be a turning point for public opinion about the Vietnam War.

Yet, most of what we know about the event comes from a single, widely publicised court martial in 1970-71.

A young Lieutenant – William Calley – in Charlie Company was tried and convicted of murdering 22 “oriental human beings” in My Lai on that sunny morning in 1968.

Forgotten tapes

Media attention on Lt Calley’s trial was extensive and the glare of publicity so bright it hid the wider, more awful truth.

Before that trial got under way, the United States army had, behind closed doors, completed an investigation of its own into the events at My Lai, and specifically into the possibility that those in authority had deliberately covered up a massacre.

Convened on 1 December 1969 in the basement of the Pentagon, The Department of the Army Review of the Preliminary Investigations into The My Lai Incident, known in abbreviated form as The Peers Inquiry, was chaired by Lt Gen William ‘Ray’ Peers.

In just 14 weeks, the Peers Inquiry conducted a comprehensive and wide-ranging investigation into the events of 16 March.

More than 400 witnesses were interviewed, and their testimony was tape-recorded.

When the inquiry concluded on 15 March 1970, those recordings were boxed-up, stored and forgotten.

In 1987, they were shipped to the US National Archives, as one small portion of a massive group of records of US Army activities in Vietnam.

There they remained hidden, never catalogued, never investigated, never uncovered – until last year.

I spent many months trying to track down the tapes.

Again and again, I was told they did not exist, but after much persistence, 48 hours of recordings from the key witnesses were declassified and made available to me.

And on 15 March, on the eve of the 40th anniversary of the massacre, some of the most powerful testimony will be broadcast for the first time, on the Archive Hour on BBC Radio 4.

Some of the interviewees’ statements reveal the mentality of the soldiers involved in the massacre.

“I would say that most people in our company didn’t consider the Vietnamese human… A guy would just grab one of the girls there and in one or two incidents they shot the girls when they got done,” said Dennis Bunning.

“That day it was just a massacre. Just plain right out, wiping out people,” said Leonard Gonzales.

“Kill everything”

The wider, more awful truth that Gen Peers uncovered, was that this was an illegal operation, planned and co-ordinated at Task Force level by Lt Col Frank Barker.

It wiped out not one but three villages: My Lai, Binh Tay and My Khe.

And not one, but two companies were involved: Bravo and Charlie.

Both of these companies were given the same briefing by their respective commanding officers, permitting them “to kill everything and anything.”

“It’s not just the people of Task Force Barker that are on trial… It’s the Army, it’s you and it’s me… and it includes our country and our people in the eyes of the world,” said Gen Peers, during his investigation.

He concluded that 30 senior officers had been negligent in their duty.

After the inquiry, 14 officers were charged with crimes.

But the only participant convicted of anything at My Lai was Lt William Calley.

Gen Peers also proposed new methods of training soldiers, guidelines for the treatment of civilians in wartime and new army leadership criteria.

His recommendations still influence today’s army training manuals.

“The My Lai Tapes” are a record not only of atrocity writ large but also of heroism.

They are a record of how war can bring out not only the worst but also the best in people.

Above all they are a record of lessons learned 40 years ago, in My Lai, Binh Tay and My Khe – lessons that should not be forgotten.

Celina Dunlop is picture editor of the Economist. The Archive Hour: The My Lai Tapes will be broadcast on BBC Radio 4 at 2000 GMT on Saturday, 15 March, 2008.

You can also listen online for seven days after that at Radio 4’s Listen again page.