In the fall of 1945, a few months after the United States dropped two atomic bombs on Japan, New Yorker writer John Hersey went to lunch with his editor, the legendary William Shawn, to discuss story ideas.

One of them would change the world.

Hersey and Shawn suspected that the U.S. government’s wartime propaganda machine had covered up the human suffering of the atomic bombs that leveled Hiroshima and Nagasaki 75 years ago this month. Pictures from Japan showed destroyed buildings and decimated neighborhoods, but little was known about the human toll, especially from radiation.

The U.S. government controlled access to the bomb sites. The War Department quietly asked American news outlets to limit information about nuclear aspects of the attacks. When reports of widespread suffering from radiation began to emerge from international journalists and Japanese officials, the American government downplayed it all as propaganda. One general even told Congress that dying from radiation was, in fact, “a very pleasant way to die.”

It was time, Hersey and Shawn decided, to find out the truth.

Hersey was 32. He had just won the Pulitzer Prize for his World War II novel “A Bell for Adano.” He traveled to Hiroshima and spent two weeks reporting the misery from the point of view of six survivors. His 30,000-word account, told in a harrowing narrative using the tools of a novelist, took up an entire issue of the New Yorker in August 1946, stirring outrage throughout the world.

The survivors, Hersey wrote, “still wonder why they lived when so many others died. Each of them counts many small items of chance or volition — a step taken in time, a decision to go indoors, catching one streetcar instead of the next — that spared him. And now each knows that in the act of survival he lived a dozen lives and saw more death than he ever thought he would see.”

Radio hosts read the piece on air. Newspapers reprinted the story and ran editorials urging readers to read it. “The public has never been told exactly what took place at Hiroshima,” an editorial in the Bee, a Danville, Va., paper, said. “It is indeed a frightful chronicle — a story which burns its way into the soul, and which makes every human being, pagan or Christian cry out — ‘this must not happen again.’ ”

Hersey’s story, later published as a book, has been celebrated as a journalistic and historical masterpiece. A panel of journalists and critics ranked it first on a list of the top 100 works of journalism in the 20th century, two slots ahead of The Washington Post’s takedown of President Richard M. Nixon. Many historians and foreign policy experts say its impact was profound enough to help prevent future use of nuclear weapons.

“In 1946, Hersey’s story was the first truly effective, internationally heeded warning about the existential threat that nuclear arms posed to civilization,” Lesley M.M. Blume writes in her new book, “Fallout: The Hiroshima Cover-Up and the Reporter Who Revealed It to the World.” “It has since helped motivate generations of activists and leaders to work to prevent nuclear war, which would likely end the brief human experiment on earth.”

In her book, Blume tells the story of how Hersey landed the story — a tale of wits, lucky breaks and hubris (the government’s, not the writer’s) that has largely gone untold over the years, mostly owing to Hersey’s desire to not talk publicly about his work and let the writing speak for itself.

A few days before his story appeared in the New Yorker, Hersey quietly disappeared to Blowing Rock, N.C., to avoid the media. Even today, his children will not speak about their father’s work, honoring his wish that “no biography of him should be written,” according to biographer Jeremy Treglown, the author of “Mr. Straight Arrow: The Career of John Hersey, Author of Hiroshima.”

Hersey was the son of Protestant missionaries. Though tall and movie-star handsome, he was loath to take himself too seriously or attract much attention. Before his Hiroshima reporting, Hersey had written several accounts of war — fiction and nonfiction, including a New Yorker piece titled “Survivor” about the Japanese sinking of future president John F. Kennedy’s Navy patrol boat.

Those pieces made him, as Blume wrote, the perfect “Trojan Horse” to tell a story the U.S. military did not want told. By the time Hersey arrived in Hiroshima, the military had been successful, Blume argues, in burying and obfuscating the true power of atomic weapons. Reporters had either “toed the line” for the war effort or moved on to other stories.

“The government junkets, conferences, speeches, and suppressed reporting were having the desired effect: across the United States, protests and alarm had been subdued to a manageable murmur,” Blume wrote. “The idea of the atomic bomb as a reasonable mainstay weapon in the national arsenal — and a nuclear future in general — was becoming acceptable to the increasingly apathetic public.”

The U.S. government, which controlled access to Hiroshima during the American occupation, had in many ways been lulled into complacency.

When Hersey applied “for clearance to travel to Hiroshima,” Blume wrote, “there may have been some bemusement about why a reporter of his stature was asking to visit the site of the nearly year-old story. Yet the perception that the press corps was so subdued and distracted at this point — and that the story had been so successfully contained — probably worked to Hersey’s advantage.”

With permission in hand, Hersey sent word to Shawn that he was on his way. He was granted a 14-day stay. “Once there,” Blume wrote, “he would have to work fast.”

Luckily, he had a blueprint for his story in mind. A few months earlier, while sick with the flu, Hersey read Thornton Wilder’s novel “The Bridge of San Luis Rey,” a tale of five strangers who die in a bridge collapse. In the novel, Wilder tells the individual stories of each character and what led them to the bridge that day.

“As Hersey read feverishly,” Blume wrote, “he realized that this would be an effective way to tackle Hiroshima as a subject,” further explaining that:

It was time for someone to describe the bomb in terms that the human mind could grasp. As Hersey finished The Bridge of San Luis Rey, he realized that emphasizing minutiae, not grandeur, was the way to drive the point home. Not everyone could comprehend how the atomic bomb worked or visualize an all-out, end-of-days nuclear world war. But practically anyone could comprehend a story about a handful of regular people — mothers, fathers, grade school children, doctors, clerks — going about their daily routines when catastrophe struck. Hersey would take readers into the victims’ kitchens, on their streetcar commutes, into their offices, back on that sunny summer morning of August 6, 1945, and show what befell them.

One of them was a Methodist minister named Kiyoshi Tanimoto.

He was away from home when the bomb was dropped. Hersey wrote:

Mr. Tanimoto, fearful for his family and church, at first ran toward them by the shortest route, along Koi Highway. He was the only person making his way into the city; he met hundreds and hundreds who were fleeing, and every one of them seemed to be hurt in some way. The eyebrows of some were burned off and skin hung from their faces and hands. Others, because of pain, held their arms up as if carrying something in both hands. Some were vomiting as they walked. Many were naked or in shreds of clothing. On some undressed bodies, the burns had made patterns — of undershirt straps and suspenders and, on the skin of some women (since white repelled the heat from the bomb and dark clothes absorbed it and conducted it to the skin), the shapes of flowers they had had on their kimonos. Many, although injured themselves, supported relatives who were worse off. Almost all had their heads bowed, looked straight ahead, were silent, and showed no expression whatever.

Tanimoto and his wife had just had a baby eight months earlier. He raced through the city trying to get home. He found them alive. Hersey described the moment:

She told him that she had got home from her night in Ushida just in time for the explosion; she had been buried under the parsonage with the baby in her arms. She told how the wreckage had pressed down on her, how the baby had cried. She saw a chink of light, and by reaching up with a hand, she worked the hole bigger, bit by bit. After about half an hour, she heard the crackling noise of wood burning.

That baby grew up and became a peace activist. Her name is Koko Kondo. (In his story, Hersey mistakenly refers to her as a boy, which he later corrected when he signed her book 40 years later, scratching out the word “son” and replacing it with “daughter.”)

As the 75th anniversary approached, Kondo, who is now 75 and living not far from Hiroshima, was thinking about Hersey, his book and what the world learned from it. Of course, she was also thinking about how the bombing had transformed her own life. The radiation left her unable to have children. Today, in addition to advocating for peace around the world, Kondo works with Japanese orphans.

“I’m so glad John Hersey wrote that book because that is what happened,” Kondo said in an interview.

But she also worries that it has been forgotten. One piece of evidence in favor of that conclusion is that nuclear weapons still exist.

More countries want them.

“We have to say to the whole world that we cannot have these powerful weapons,” Kondo said. “If we use these weapons again, it will be the end of the whole world.”

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