At the end of this month 70 years will have passed since the publication of a magazine story hailed as one of the greatest pieces of journalism ever written. Headlined simply Hiroshima, the 30,000-word article by John Hersey had a massive impact, revealing the full horror of nuclear weapons to the post-war generation, as Caroline Raphael describes.

I have an original copy of the 31 August 1946 edition of The New Yorker. It has the most innocuous of covers - a delightful playful carefree drawing of summer in a park. On the back cover, the managers of the New York Giants and the New York Yankees encourage you to "Always Buy Chesterfield" cigarettes.

Past the Goings on About Town and movie listings, past the ritzy adverts for diamonds and fur and cars and cruises you find a simple statement from The Editors explaining that this edition will be devoted entirely to just one article "on the almost complete obliteration of a city by one atomic bomb". They are taking this step, they say, "in the conviction that few of us have yet comprehended the all but incredible destructive power of this weapon, and that everyone might well take time to consider the terrible implications of its use".

Seventy years ago no-one talked about stories "going viral", but the publication of John Hersey's article Hiroshima in The New Yorker achieved just that. It was talked of, commented on, read and listened to by many millions all over the world as they began to understand what really happened not just to the city but to the people of Hiroshima on 6 August 1945 and in the following days.

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It was spring 1946 when John Hersey, decorated war correspondent and prize-winning novelist, was commissioned by The New Yorker to go to Hiroshima. He expected to write, as others had done, a piece about the state of the shattered city, the buildings, the rebuilding, nine months on.

Readers who sent letters to The New Yorker wrote of their shame and horror that ordinary people, just like them - secretaries and mothers, doctors and priests - had endured such terror

On the voyage out he fell ill and was given a copy of Thornton Wilders's The Bridge of San Luis Rey. Inspired by Wilder's narrative of the five people who crossed the bridge as it collapsed he decided he would write about people not buildings. And it was that simple decision that marks Hiroshima out from other pieces of the time. Once in Hiroshima he found survivors of the bomb whose stories he would tell, starting from the minutes before the bomb was dropped. Many years later he told of the horror he felt, how he could only stay a few weeks.

Hersey took these accounts back to New York. Had he filed from Japan the chances of them ever being published would have been remote - previous attempts to get graphic photographs or film or reports out of the country had been halted by the US Occupying Forces. The material had been censored or locked away - sometimes it simply disappeared.

John Hersey - 1914-1993

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Born in China, the son of US missionaries

Returned to the US aged 10, later studied at Yale

Began writing for Time in 1937, reported from Europe and Asia during the war

His first novel, A Bell for Adano (1944) - about a Sicilian town occupied by US forces - won a Pulitzer Prize

Hiroshima tops one list of the best 20th Century American journalism

Hersey's editors, Harold Ross and William Shawn, knew they had something quite extraordinary, unique, and the edition was prepared in utter secrecy. Never before had all the magazine's editorial space been given over to a single story and it has never happened since. Journalists who were expecting to have their stories in that week's edition wondered where their proofs had gone. Twelve hours before publication, copies were sent to all the major US newspapers - a smart move that resulted in editorials urging everyone to read the magazine.

It was read out over four consecutive nights on the new Third Programme, despite some concern among senior managers about the emotional impact on listeners

All 300,000 copies immediately sold out and the article was reprinted in many other papers and magazines the world over, except where newsprint was rationed. When Albert Einstein attempted to buy 1,000 copies of the magazine to send to fellow scientists he had to contend with facsimiles. The US Book of the Month Club gave a free special edition to all its subscribers because, in the words of its president, "We find it hard to conceive of anything being written that could be of more important at this moment to the human race." Within two weeks a second-hand copy of The New Yorker sold for 120 times its cover price.

If Hiroshima demonstrates anything as a piece of journalism it is the enduring power of storytelling. John Hersey combined all his experience as a war correspondent with his skill as a novelist.

It was a radical piece of journalism that gave a vital voice to those who only a year before had been mortal enemies. There in a cataclysmic landscape of living nightmares, of the half-dead, of burnt and seared bodies, of desperate attempts to care for the blasted survivors, of hot winds and a flattened city ravaged by fires we meet Miss Sasaki , the Rev Mr Tanimoto, Mrs Nakamura and her children, the Jesuit Father Kleinsorge and doctors Fujii and Sasaki.

The six characters

Miss Toshiko Sasaki - personnel department clerk aged about 20 who was 1,600 yards from the centre of the blast, her leg is horribly injured

The Rev Mr Kiyoshi Tanimoto - pastor of the Hiroshima Methodist Church, falls ill from radiation sickness

Mrs Hatsuyo Nakamura - the widow of a tailor who died serving in Singapore, with children aged 10 and below

Father Wilhelm Kleinsorge - a German Jesuit priest who feels the strain of being a foreigner in Japan and suffers from exposure to radiation

Doctors Masakazu Fujii and Terufumi Sasaki (not related to Miss Sasaki) - two temperamentally very different medics

There had been demonisation long before Pearl Harbor. The Yellow Peril of the cartoon strips had sunk deep into the American psyche. In 1941 Time-Life ran an extraordinary article telling readers how they could tell Japanese from Chinese - "How to tell your friends from the Japs". The pilot of the Enola Gay is reported to have said he felt like sci-fi hero Buck Rogers the day he dropped the bomb.

So only a year after the end of the war these six close-ups on five Japanese men and women and one Westerner, each of whom "saw more death than he ever thought he would see" were unexpected and shattering. Readers who sent letters to The New Yorker, almost all in admiration for the work, wrote of their shame and horror that ordinary people, just like them - secretaries and mothers, doctors and priests - had endured such terror.

John Hersey was not the first to report from Hiroshima but the reports and newsreels had been a blizzard of numbers too big to fully comprehend. They had reported on the destruction of the city, the mushroom cloud, the shadows of the dead on the walls and streets but never got close to those who lived through those end-of-days time, as Hersey did.

It was also becoming increasingly clear to some that this new weapon carried on killing long after the "noiseless flash" as bright as the sun, despite intense government and military attempts to cover it up or deny it.

Image caption The book has never been out of print

Hiroshima was the first publication to make the man on the San Francisco trolleybus and the woman on the Clapham omnibus confront the miseries of radiation sickness, to understand that you could survive the bomb and still die from its after effects. John Hersey in his calm unflinching prose reported what those who had survived had witnessed. As the nuclear arms race began, just three months after the testing of further atom bombs at Bikini Atoll, the true power of the new weapons began to be understood.

Such were the reverberations of Hersey's article, and Albert Einstein's very public support for it, that Henry Stimson who had been US Secretary for War wrote a magazine article in reply, The Decision to Use the Atomic Bomb - a defiant justification for the use of the bomb, whatever the consequences.

News of the extraordinary article had been reported in Britain, but it was too long to publish - John Hersey would not allow it to be edited and newsprint was still rationed. So the BBC followed American radio's lead and about six weeks later it was read out over four consecutive nights on the new Third Programme, despite some concern among senior managers about the emotional impact on listeners.

The Radio Times commissioned Alistair Cooke to write a long background piece. Alluding to its publication in The New Yorker, renowned as the home of witty cartoons, he called it "the deadliest joke of our age".

Find out more

Listen to Hersey's Hiroshima at 23:00 on Monday 22 August, on BBC Radio 4, or catch up afterwards on the BBC iPlayer

at 23:00 on Monday 22 August, on BBC Radio 4, or catch up afterwards on the BBC iPlayer The 1948 reading of Hiroshima will be played in four parts, at 18:30 from Tuesday 23 to Friday 26 August, on Radio 4 Extra - you can catch up on the iPlayer here

The listening figures were high and the BBC decided to rebroadcast the reading on the Light Programme all in one go, just a few weeks later, to make sure even more people heard it. That's the Light Programme whose remit was, according to the BBC Handbook for that year, "to entertain its listeners and to interest them in the world at large without failing to be entertaining". There was little to entertain in this two-hour programme. The Daily Express critic, Nicholas Hallam, called it the most terrifying broadcast he had ever heard.

Hersey never forgot his survivors - in 1985, on the 40th anniversary of the bomb, he went back to Japan

The BBC had also invited John Hersey to be interviewed and his cabled reply is in the BBC archives:

"Hersey gratefullest invitation and BBC interest and coverage Hiroshima but has throughout maintained policy let story speak for itself without additional words from himself or anybody."

Indeed, Hersey was only to give three or four interviews his entire life. Sadly not one of them was for the BBC.

A 1948 recording of a reading of Hiroshima remains in the BBC archives. The effect of the crisp English voices telling this harrowing story is startling. The prose is revealed as rhythmic and often quietly poetic and ironic. One of the readers is the young actress Sheila Sim, newly married at the time to the actor Richard Attenborough.

By November, Hiroshima was published in book form. It was translated quickly into many languages and a braille edition was released. However, in Japan, Gen Douglas MacArthur - the supreme commander of occupying forces, who effectively governed Japan until 1948 - had strictly prohibited dissemination of any reports on the consequences of the bombings. Copies of the book, and the relevant edition of The New Yorker, were banned until 1949, when Hiroshima was finally translated into Japanese by the Rev Mr Tanimoto, one of Hersey's six survivors.

Hersey never forgot his survivors. In 1985, on the 40th anniversary of the bomb, he went back to Japan and wrote The Aftermath, the story of what had happened to them in the intervening four decades. Two of them had since died, one of them certainly from radiation-related disease.

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