On the eve of the Arab-Israeli war, 50 years ago this week, Israeli officials raced to assemble an atomic device and developed a plan to detonate it atop a mountain in the Sinai Peninsula as a warning to Egyptian and other Arab forces, according to interviews with a key organiser of the effort that were published last Monday.

The secret contingency plan, called a “doomsday operation” by Itzhak Yaakov, the retired brigadier general who described it in the interviews, would have been invoked if Israel feared it was going to lose the 1967 conflict. The demonstration blast, Israeli officials believed, would intimidate Egypt and surrounding Arab states — Syria, Iraq and Jordan — into backing off.

Israel won the war so quickly that the atomic device was never moved to Sinai. But Yaakov’s account, which sheds new light on a clash that shaped the contours of the modern Middle East conflict, reveals Israel’s early consideration of how it might use its nuclear arsenal to preserve itself.

“It’s the last secret of the 1967 war,” said Avner Cohen, a leading scholar of Israel’s nuclear history who conducted the interviews. Yaakov, who oversaw weapons development for the Israeli military, detailed the plan to Cohen in 1999 and 2000, years before he died in 2013 at 87.

“Look, it was so natural,” said Yaakov, according to a transcription of a taped interview. “You’ve got an enemy, and he says he’s going to throw you to the sea. You believe him.”

“How can you stop him?” he asked. “You scare him. If you’ve got something you can scare him with, you scare him.”

Israel has never acknowledged the existence of its nuclear arsenal, in an effort to preserve “nuclear ambiguity” and forestall periodic calls for a nuclear-free Middle East. In 2001, Yaakov was arrested, aged 75, on charges that he had imperiled the country’s security by talking about the nuclear programme to an Israeli reporter, whose work was censored. At various moments, US officials, including former president Jimmy Carter long after he left office, have acknowledged the existence of the Israeli programme, though they have never given details.

A spokesman for the Israeli embassy in Washington said the Israeli government would not comment on Yaakov’s role.

If the Israeli leadership had detonated the atomic device, it would have been the first nuclear explosion used for military purposes since the US attacks on Hiroshima and Nagasaki 22 years earlier.

The plan had a precedent: the US considered the same thing during the Manhattan Project, as the programme’s scientists hotly debated whether to set off a blast near Japan in an effort to scare Emperor Hirohito into a quick surrender. The military vetoed the idea, convinced that it would not be enough to end the war.

According to Yaakov, the Israeli plan was code-named Shimshon, or Samson, after the biblical hero of immense strength. Israel’s nuclear deterrence strategy has long been called the “Samson option” because Samson brought down the roof of a Philistine temple, killing his enemies and himself. Yaakov said he feared that if Israel, as a last resort, went ahead with the demonstration nuclear blast in Egyptian territory, it could have killed him and his commando team.

Cohen, a professor at the Middlebury Institute of International Studies at Monterey and the author of Israel and the Bomb and The Worst-Kept Secret, described the idea behind the atomic demonstration as giving “the prime minister an ultimate option if everything else failed”. Cohen, who was born in Israel and educated in part in the US, has pushed the frontiers of public discourse on a fiercely hidden subject: how Israel became an unacknowledged nuclear power in the 1960s.

On Monday, the Nuclear Proliferation International History Project of the Woodrow Wilson International Centre for Scholars in Washington — where Cohen is a global fellow — released on a special website a series of documents related to the atomic plan. The project maintains a digital archive of his work known as the Avner Cohen Collection. (President Donald Trump’s proposed budget calls for the elimination of all federal funding for the centre, which Congress created as a living memorial to Wilson.)

It has long been known that Israel, fearful for its existence, rushed to complete its first atomic device on the eve of the Arab-Israeli war. But the planned demonstration remained secret in a country where it is taboo to discuss even half-century-old nuclear plans, and where fears persist that Iran will eventually obtain a nuclear weapon, despite its deal with world powers.

Itzhak Yaakov never fully recovered from his legal ordeal

Shimon Peres, the former Israeli president and prime minister who died last year, hinted at the plan’s existence in his memoirs. He referred to an unnamed proposal that “would have deterred the Arabs and prevented the war”.

At the time of the 1967 war, the world’s main nuclear powers were observing an accord known as the Partial Test Ban Treaty. To curb radiation hazards, it prohibited all test detonations of nuclear arms except for those conducted underground. That Israel considered an open explosion was a measure of its desperation.

“The goal,” Yaakov says on the transcribed tape, “was to create a new situation on the ground, a situation which would force the great powers to intervene, or a situation which would force the Egyptians to stop and say, ‘Wait a minute, we didn’t prepare for that.’ The objective was to change the picture.”

Cohen said he struck up a relationship with Yaakov after he published Israel and the Bomb in 1998. He interviewed him for hours in the summer and autumn of 1999 and in early 2000, always in Hebrew and mainly in midtown Manhattan, where the former general lived.

Those interviews paint a picture of Israel’s recognition in the early 1960s that it needed a crash programme to get the bomb. In 1963, Yaakov, a freshly minted colonel with engineering degrees from the Massachusetts Institute of Technology and from Technion, the Israel Institute of Technology, became the senior liaison officer between the Israel Defence Forces and the country’s civilian defence units, including the project to make an atomic bomb.

As Yaakov recounted the story, in May 1967, as tensions rose with Egypt over its decision to close the Straits of Tiran between the Gulf of Aqaba and the Red Sea, he was half a world away, visiting the Rand Corp, in California. He was suddenly summoned back to Israel. With it clear that war was imminent, Yaakov said, he initiated, drafted and promoted a plan aimed at detonating a nuclear device in the sparsely populated Eastern Sinai Desert in a display of force.

The site chosen for the proposed explosion was a mountain top about 12 miles from an Egyptian military complex at Abu Ageila, a critical crossroads where, on June 5, Ariel Sharon commanded Israeli troops in a battle against the Egyptians. (Sharon later became prime minister, and died in 2014.)

The plan, if activated by order of the prime minister and military chief of staff, was to send a small paratrooper force to divert the Egyptian army in the desert area so that a team could lay preparations for the atomic blast. Two large helicopters were to land, deliver the nuclear device, and then create a command post in a mountain creek or canyon. If the order came to detonate, the blinding flash and mushroom cloud would have been seen throughout the Sinai and Negev deserts, and perhaps as far away as Cairo.

Yaakov described a helicopter reconnaissance flight he made with Israel Dostrovsky, the first director general of the Israel Atomic Energy Commission, the civilian arm of the bomb programme. The helicopter had to turn back after the pilots learnt that Egyptian jets were taking off, perhaps to intercept them. “We got very close,” Yaakov recalled. “We saw the mountain, and we saw that there is a place to hide there, in some canyon.”

On the eve of the war, Yaakov said, he was filled with the same doubts that had gnawed at the American scientists during the Manhattan Project. Would the bomb explode? Would he survive the blast? He never got to find out. Israel defeated three Arab armies, gained territory four times its original size and became the region’s foremost military power using conventional arms.

Nonetheless, Yaakov continued to lobby for an atomic demonstration to make clear the country’s new status as a nuclear power. But the idea went nowhere. “I still think to this day that we should have done it,” he told Cohen.

During a visit to Israel, a year after telling his story to Cohen in New York, where he had worked as a venture capitalist after having played a key role in the founding of Israel’s technology industry, Yaakov was arrested on charges of “high espionage” that carried a maximum penalty of life behind bars. The exact charges were a mystery, and he was put on a secret trial.

“We see this as a very sad story of a person who dedicates his life to the security of Israel and ends up caught in a huge story that gets blown out of proportion and jeopardises his reputation, his career, his legacy, everything,” Jack Chen, one of his lawyers, told The New York Times at the time.

It turned out that the charges centred on his conversations with an Israeli reporter, whose account of the 1967 plan was censored by the military. Yaakov was found guilty of handing over secret information without authorisation, the lesser of the charges against him. He was given a two-year suspended sentence.

The Israeli newspaper Haaretz, in its obituary of Yaakov, said he had never fully recovered from his legal ordeal and, during his final days, bitterly discussed its details with fellow retired officers.

Cohen said he and Yaakov continued to get together long after the interviews and the secret trial — for instance, in a restaurant in Tel Aviv around 2009. He said he had promised Yaakov he would find the right time and the right place to make his story public. Now, he said, on the 50th anniversary of the war — with Yaakov and so many other witnesses long dead — it seemed like the right time.