Three decades ago, the world was on the brink of a nuclear showdown – until Jimmy Carter showed up in North Korea.

In June 1994, the former US president arrived for talks in Pyongyang with then leader Kim Il-sung. It was unprecedented, marking the first time a former or sitting US president had visited.

But it was also an extraordinary act of personal intervention, one which many believe narrowly averted a war between the US and North Korea that could have cost millions of lives. And it led to a period of greater engagement between Pyongyang and the West.

All this may not have happened if not for a set of diplomatic chess moves by Carter, who died aged 100 on 29 December.

“Kim Il-sung and Bill Clinton were stumbling into a conflict, and Carter leapt into the breach, successfully finding a path for negotiated resolution of the standoff,” North Korean expert John Delury, of Yonsei University, told the BBC.

In early 1994, tensions were running high between Washington and Pyongyang, as officials tried to negotiate an end to North Korea’s nuclear programme.

US intelligence agencies suspected that despite ongoing talks, North Korea may have secretly developed nuclear weapons.

Then, in a startling announcement, North Korea said it had begun withdrawing thousands of fuel rods from its Yongbyon nuclear reactor for reprocessing. This violated an earlier agreement with the US under which such a move required the presence of inspectors from the International Atomic Energy Agency (IAEA) nuclear watchdog.

North Korea also announced it would withdraw from the IAEA.

American suspicion spiked as Washington believed Pyongyang was preparing a weapon, and US officials broke off negotiations. Washington began preparing several retaliatory measures, including initiating UN sanctions and reinforcing troops in South Korea.

In subsequent interviews, US officials revealed they also contemplated dropping a bomb or shooting a missile at Yongbyon – a move which they knew would have likely resulted in war on the Korean peninsula and the destruction of the South’s capital, Seoul.

It was in this febrile atmosphere that Carter made his move.

For years, he had been quietly wooed by Kim Il-sung, who had sent him personal entreaties to visit Pyongyang. In June 1994, upon hearing Washington’s military plans, and following discussions with his contacts in the US government and China – North Korea’s main ally – Carter decided to finally accept Kim’s invitation.

“I think we were on the verge of war,” he told the US public broadcaster PBS years later. “It might very well have been a second Korean War, within which a million people or so could have been killed, and a continuation of the production of nuclear fissile material… if we hadn’t had a war.”

Carter’s visit was marked by skillful diplomatic footwork – and brinkmanship.

First, Carter had to test Kim’s sincerity. He made a series of requests, all of which were agreed to, except the last: Carter wanted to travel to Pyongyang from Seoul across the demilitarised zone (DMZ), a strip of land that acts as a buffer between the two Koreas.

“Their immediate response was that no-one had ever done this for the last 43 years, that even the United Nations secretary-general had to go to Pyongyang through Beijing. And I said, ‘Well, I’m not going, then’,” he said.

A week later, Kim caved.

The next step for Carter was harder – convincing his own government to let him go. Robert Gallucci, the chief US negotiator with North Korea at the time, later said there was “discomfort in almost all quarters” about the US essentially “subcontracting its foreign policy” to a former president.

Carter first sought permission from the State Department, who blanked him. Unfazed, he decided to simply inform then-US president Bill Clinton that he was going, no matter what.

He had an ally in vice-president Al Gore, who intercepted Carter’s communication to Clinton. “[Al Gore] called me on the phone and told me if I would change the wording from “I’ve decided to go” to “I’m strongly inclined to go” that he would try to get permission directly from Clinton… he called me back the next morning and said that I had permission to go.”

The trip was on.

On 15 June 1994, Carter crossed over to North Korea, accompanied by his wife Rosalyn, a small group of aides and a TV crew.

Meeting Kim was a moral dilemma for Carter.

“I had despised Kim Il-sung for 50 years. I was in a submarine in the Pacific during the Korean War, and many of my fellow servicemen were killed in that war, which I thought was precipitated unnecessarily by him,” he told PBS.

“And so I had very serious doubts about him. When I arrived, though, he treated me with great deference. He was obviously very grateful that I had come.”

Over several days, the Carters had meetings with Kim, were taken on a sightseeing tour of Pyongyang and went on a cruise on a luxury yacht owned by Kim’s son, Kim Jong-il.

Carter discovered his hunch was right: North Korea not only feared a US military strike on Yongbyon, but was also ready to mobilise.

“I asked [Kim’s advisers] specifically if they had been making plans to go to war. And they responded very specifically, ‘Yes, we were’,” he said.

“North Korea couldn’t accept the condemnation of their country and the embarrassment of their leader and that they would respond.

“And I think this small and self-sacrificial country and the deep religious commitments that you had, in effect, to their revered leader, their Great Leader as they called him, meant that they were willing to make any sacrifice of massive deaths in North Korea in order to preserve their integrity and their honour, which would have been a horrible debacle in my opinion.”

Carter presented a list of demands from Washington as well as his own suggestions. They included resuming negotiations with the US, starting direct peace talks with South Korea, a mutual withdrawal of military forces, and helping the US find remains of US soldiers buried in North Korean territory.

“He agreed to all of them. And so, I found him to be very accommodating,” Carter said. “So far as I know then and now, he was completely truthful with me.”

Crucially, Carter came up with a deal where North Korea would stop its nuclear activity, allow IAEA inspectors back into its reactors, and eventually dismantle Yongbyon’s facilities. In return, the US and its allies would build light-water reactors in North Korea, which could generate nuclear energy but not produce material for weapons.

While enthusiastically embraced by Pyongyang, the deal was met with reluctance from US officials when Carter suggested it in a phone call. He then told them he was going on CNN to announce details of the deal – leaving the Clinton administration little choice but to agree.

Carter would later justify forcing his own government’s hand by saying he had to “consummate a resolution of what I considered to be a very serious crisis”. But it did not go down well back home – officials were unhappy at Carter’s “freelancing” and attempt to “box in” Clinton, according to Mr Gallucci.

Near the end of the trip, they told him to convey a statement to the North Koreans, reiterating Clinton’s public position that the US was continuing to press for UN sanctions. Carter disagreed, according to reports at that time.

Hours later, he got on the boat with Kim, and promptly went off-script. As TV cameras rolled, he told Kim the US had stopped work on drafting UN sanctions – directly contradicting Clinton.

An annoyed White House swiftly disowned Carter. Some openly expressed frustration, painting a picture of a former president going rogue. “Carter is hearing what he wants to hear… he is creating his own reality,” a senior official complained at the time to The Washington Post.

Many in Washington also criticised him for the deal itself, saying the North Koreans had used him.

But Carter’s savvy use of the news media to pressure the Clinton administration worked. By broadcasting his negotiations almost instantaneously, he gave the US government little time to react, and immediately after his trip “it was possible to see an almost hour-by-hour evolution in US policy towards North Korea” where they ratcheted down their tone, wrote CNN reporter Mike Chinoy who covered Carter’s trip.

Though Carter later claimed he had misspoken on the sanctions issue, he also responded with typical stubbornness to the blowback.

“When I got back to Seoul, I was amazed and distressed at the negative reaction that I had from the White House. They urged me not to come to Washington to give a briefing, urged me to go directly to… my home,” he said.

But he went against their wishes.

“I decided that what I had to offer was too important to ignore.”

A final dramatic coda to the episode happened a month later.

On 9 July 1994, on the same day as US and North Korean officials sat down in Geneva to talk, state media flashed a stunning announcement: Kim Il-sung had died of a heart attack.

Carter’s deal was immediately plunged into uncertainty. But negotiators ploughed through, and weeks later hammered out a formal plan known as the Agreed Framework.

Though the agreement broke down in 2003, it was notable for freezing Pyongyang’s nuclear programme for nearly a decade.

Robert Carlin, a former CIA and US state department official who led delegations in negotiations with North Korea, noted that Carter’s real achievement was in getting the US government to co-operate.

“Carter was, more or less, pushing on an open door in North Korea. It was Washington that was the bigger challenge… if anything, Carter’s intervention helped stop the freight train of US decision-making that was hurtling toward a cliff,” he told the BBC.

Carter’s visit was also significant for opening a path for rapprochement, which led to several trips later, including one in 2009 when he travelled with Clinton to bring home captured US journalists.

He is also credited with paving the way for Donald Trump’s summit with Kim Jong Un – Kim Il-sung’s grandson – in 2018, as “Carter made it imaginable” that a sitting US president could meet with a North Korean leader, Dr Delury said.

That summit failed, and of course, in the long run Carter’s trip did not succeed in removing the spectre of nuclear war, which has only grown – these days North Korea has missiles regarded as capable of hitting the US mainland.

But Carter was lauded for his political gamble. It was in sharp contrast to his time in office, when he was criticised for being too passive on foreign policy, particularly with his handling of the Iran hostage crisis.

His North Korea trip “was a remarkable example of constructive diplomatic intervention by a former leader,” Dr Delury said.

His legacy is not without controversy, given the criticism that he took matters in his own hands. His detractors believe he played a risky and complicated game by, as CNN’s Mike Chinoy put it, “seeking to circumvent what he viewed as a mistaken and dangerous US policy by pulling the elements of a nuclear deal together himself”.

But others believe Carter was the right man for the job at the time.

He had “a very strong will power”, but was also “a man of peace inside and out,” said Han S Park, one of several people who helped Carter broker the 1994 trip.

Though his stubbornness also meant that he “did not get along with a lot of people”, ultimately this combination of attributes meant he was the best person “to prevent another occurrence of a Korean War”, Prof Park said.

More than anything, Carter was convinced he was doing the right thing.

“He didn’t let US government clucking and handwringing stop him,” says Robert Carlin. “Carter had guts.”