Lee Saedol was the finest Go player of his generation when he suffered a decisive loss, defeated not by a human opponent but by artificial intelligence.

Mr. Lee was beaten by AlphaGo, an A.I. computer program developed by Google’s DeepMind unit. The stunning upset, in 2016, made headlines around the world and looked like a clear sign that artificial intelligence was entering a new, profoundly unsettling era.

By besting Mr. Lee, an 18-time world champion revered for his intuitive and creative style of play, AlphaGo had solved one of computer science’s greatest challenges: teaching itself the abstract strategy needed to win at Go, widely considered the world’s most complex board game.

“I am very surprised because I have never thought I would lose,” Mr. Lee said at the time in a post-match news conference. “I didn’t know that AlphaGo would play such a perfect Go.”

But the implications of his loss went far beyond the game itself, in which two players compete for territory by placing black and white stones on a gridded board made up of 19 lines by 19 lines. AlphaGo’s victory demonstrated the unbridled potential of A.I. to achieve superhuman mastery of skills once considered too complicated for machines.

Mr. Lee, now 41, retired three years later, convinced that humans could no longer compete with computers at Go. Artificial intelligence, he said, had changed the very nature of a game that originated in China more than 2,500 years ago.

“Losing to A.I., in a sense, meant my entire world was collapsing,” he said in a recent interview with The New York Times.

As society wrestles with what A.I. holds for humanity’s future, Mr. Lee is now urging others to avoid being caught unprepared, as he was, and to become familiar with the technology now. He delivers lectures about A.I., trying to give others the advance notice he wishes he had received before his match.

“I faced the issues of A.I. early, but it will happen for others,” Mr. Lee said recently at a community education fair in Seoul to a crowd of students and parents. “It may not be a happy ending.”

Since his loss, Mr. Lee has become an A.I. obsessive of sorts, following with rapt if uneasy attention as artificial intelligence delivers one breakthrough after another.

A.I. has helped chatbots carry on conversations almost indistinguishable from human interaction. It has solved problems that have confounded scientists for decades like predicting protein shapes. And it has blurred the lines of creativity: writing music, producing art and generating videos.

Mr. Lee is not a doomsayer. In his view, A.I. may replace some jobs, but it may create some, too. When considering A.I.’s grasp of Go, he said it was important to remember that humans both created the game and designed the A.I. system that mastered it.

What he worries about is that A.I. may change what humans value.

“People used to be in awe of creativity, originality and innovation,” he said. “But since A.I. came, a lot of that has disappeared.”

Mr. Lee started playing Go at the age of 5 under the guidance of his father, a schoolteacher and enthusiast of the game. His family lived on Bigeumdo, an island off the southwest coast of the Korean Peninsula inhabited by around 3,600 people.

His immense talent was apparent from the start. He quickly became the best player of his age not only locally but across all of South Korea, Japan and China. He turned pro at 12.

By the time he was 20, Mr. Lee had reached 9-dan, the highest level of mastery in Go. Soon, he was among the best players in the world, described by some as the Roger Federer of the game.

“He was an idol, he was a star,” said Lee Hajin, a former professional Go player. “Everyone looked up to him,” Ms. Lee added

As Mr. Lee’s standing was growing, Go started garnering interest from a new audience: computer scientists.

Go posed a tantalizing challenge for A.I. researchers. The game is exponentially more complicated than chess, with it often being said that there are more possible positions on a Go board (10 with more than 100 zeros after it, by many mathematical estimates) than there are atoms in the universe.

The breakthrough came from DeepMind, which built AlphaGo using so-called neural networks: mathematical systems that can learn skills by analyzing enormous amounts of data. It started by feeding the network 30 million moves from high-level players. Then the program played game after game against itself until it learned which moves were successful and developed new strategies.

By late 2015, AlphaGo had defeated a three-time European Go champion five straight times in a closed-door match.

Then, Mr. Lee was approached by Ms. Lee, the former professional who was working at the International Go Federation, with a proposal for a public match, with a $1 million prize for beating AlphaGo.

Mr. Lee said he accepted the offer without much thought, figuring it would be “fun.”

“But fun with the presumption that I was going to win,” he said. “The possibility of losing didn’t occur to me.”

The best-of-five match, played in Seoul, was a spectacle. In South Korea, where millions of people play Go and Mr. Lee is a celebrity, the showdown led nightly television broadcasts. More than 200 million people watched, with huge audiences in China and Japan.

During the matches, a DeepMind engineer sat across from Mr. Lee and placed the stones as relayed to him by AlphaGo. Mr. Lee said not having a true human opponent was disconcerting. AlphaGo played a style he had never seen, and it felt odd to not try to decipher what his opponent was thinking and feeling. The world watched in awe as AlphaGo pushed Mr. Lee into corners and made moves unthinkable to a human player.

“I couldn’t get used to it,” he said. “I thought that A.I. would beat humans someday. I just didn’t think it was here yet.”

AlphaGo won 4 of 5 matches. Lee Sang Hoon, his older brother and a professional Go player, remembered thinking: “This can’t be.”

“It was shocking,” said his brother, who continues to play as a professional. Like other pros, he now trains with A.I. systems that continue to learn and improve.

“Pro players are studying how these algorithms work and are trying to close the gap,” his brother said. “But we are a long way away.”

AlphaGo’s victory “was a watershed moment in the history of A.I.” said Demis Hassabis, DeepMind’s chief executive, in a written statement. It showed what computers that learn on their own from data “were really capable of,” he said.

Mr. Lee had a hard time accepting the defeat. What he regarded as an art form, an extension of a player’s own personality and style, was now cast aside for an algorithm’s ruthless efficiency.

“I could no longer enjoy the game,” he said. “So I retired.”

Mr. Lee has kept one foot in the Go world. He has written several books, including an autobiography and a series about his famous matches. He has created Go-inspired board games. He founded a Go academy for children with about a dozen branches across the country.

But A.I. dominates his thoughts, partly because of the ambivalence he feels about the pros and cons, but also because it’s a subject that hits close to home.

His 17-year-old daughter is in her final year of high school. When they discuss what she should study at university, they often consider a future shaped by A.I.

“We often talk about choosing a job that won’t be easily replaceable by A.I. or less impacted by A.I.,” he said. “It’s only a matter of time before A.I. is present everywhere.”

Cade Metz contributed reporting from San Francisco.

Rwanda does not have to repay the hundreds of millions of pounds it received from Britain as part of a contentious policy aimed at sending migrants on a one-way flight to the Central African nation, two senior Rwandan government officials say.

Rwanda’s president had previously suggested that such money could be returned.

As part of the deal, Britain was set to give Rwanda as much as about half a billion pounds in development funding in exchange for taking in the migrants. Britain’s independent public spending watchdog said in early March that the country had already paid Rwanda £220 million, about $280 million, even though no asylum seekers had been deported to the African nation.

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A few weeks before Hamas attacked Israel on Oct. 7, a senior White House official visited eastern Lebanon for a sightseeing trip that doubled as a dramatic political statement.

The official, Amos Hochstein, one of President Biden’s most trusted national security advisers, toured the ancient ruins of Baalbek in an area well known as a stronghold of Hezbollah, a U.S.-designated terrorist group sponsored by Iran.

Wearing white pants and a golf shirt, and with no security entourage, Mr. Hochstein marveled at the artifacts and snapped photos of the onetime Roman city’s crumbling stone walls and columns. Keeping watch from a distance were several muscular men in black T-shirts — presumed Hezbollah militiamen.

The trip caused a minor sensation in Lebanese news media, which wondered how a top American official — one born in Israel, no less — was able to move so freely on Hezbollah turf.

The trip demonstrated the surprising way Mr. Hochstein has become one of the few Americans trusted, however grudgingly, by Hezbollah’s leadership. And that trust is crucial today, now that Mr. Biden has designated Mr. Hochstein as his diplomatic point man for preventing clashes across the Israel-Lebanon border from exploding into a war that could be even more devastating than the conflict in Gaza.

Officially, Mr. Hochstein, 51, is Mr. Biden’s top aide for global energy and infrastructure. But his wonky title does not capture the ever-broadening portfolio bestowed upon him by a president whose close confidence he has earned over more than a decade and who is said to view his adviser as a results-getting “doer.”

Mr. Hochstein has made at least five trips to Israel and Lebanon since the war in Gaza prompted Hezbollah to launch rocket attacks on northern Israel in solidarity with Hamas. He speaks constantly with Lebanese officials as well as top Israeli officials, sometimes including Prime Minister Benjamin Netanyahu.

“He’s a very close adviser of the president,” said Edward M. Gabriel, the president of the American Task Force on Lebanon, a nonprofit organization in Washington that seeks better relations between the United States and Lebanon. “As a consequence, I think he can speak with a lot of authority when he’s in the field.”

Last week, Mr. Hochstein, who cuts a dashing profile in his slim-fitting suits and slicked-back hair, was in Paris coordinating U.S. and French efforts to bring calm to the Israel-Lebanon border. In mid-June, he saw officials in both countries, and a week later met twice in Washington with Israel’s defense minister, Yoav Gallant, with whom he speaks on a regular basis.

In addition to his work on the Israel-Hezbollah file, Mr. Hochstein has also been one of Mr. Biden’s main envoys to Saudi Arabia. He was among the U.S. officials who helped convince Mr. Biden that the United States should not ostracize Crown Prince Mohammed bin Salman despite revulsion over the murder of the Saudi dissident and Washington Post columnist Jamal Khashoggi in 2018.

Working in tandem with a White House colleague, Brett McGurk, the top National Security Council official for Middle East affairs, he has led quiet diplomacy in pursuit of an ambitious grand bargain that would include a U.S.-Saudi security agreement and normalized diplomatic relations between Israel and Saudi Arabia. Mr. Hochstein has met with Prince Mohammed more than a dozen times, talks that have also included Saudi oil production plans. (Mr. Hochstein reports to and works closely with the president’s national security adviser, Jake Sullivan.)

A former lobbyist, congressional aide and executive at the natural gas firm Tellurian, Mr. Hochstein is passionate about renewable energy, and has trumpeted his purchase of an all-electric Ford Mustang with rooftop solar panels, although some environmental activists have complained about his background in the fossil fuel industry.

He joined the Biden administration as the State Department’s top energy official, helping to manage oil and gas market disruptions after Russia invaded Ukraine. He was reassigned to Mr. Biden’s White House staff early last year, reflecting the trust he has built with Mr. Biden over many years, including during numerous foreign trips he joined when Mr. Biden was vice president and Mr. Hochstein was a State Department energy policy official.

“President Biden likes and admires him,” said Aaron David Miller, a senior fellow with the Carnegie Endowment for International Peace, who recently hosted Mr. Hochstein for an online conversation. “Anyone who can help convince President Biden that M.B.S. should move from being a pariah to a partner — that takes a lot of lifting,” he added, referring to Prince Mohammed by his initials.

Mr. Hochstein is now in the daily thick of a simmering crisis that has become one of the Biden administration’s greatest worries: that low-grade fighting between Israel and Hezbollah could escalate into a nightmare scenario that draws Iran and the United States into the conflict more directly.

Based in southern Lebanon, Hezbollah was formed in the 1980s to resist Israel’s invasion of the country. It has developed a huge arsenal of rockets and missiles capable of inflicting enormous damage on Israel’s cities.

“There is a very active mini-war going on between Israel and Lebanon,” Mr. Hochstein said during his Carnegie Endowment talk. “Thousands of rockets have been fired from Lebanon into Israel, and thousands of rounds have been shot by Israel into Lebanon.” (The White House declined to make Mr. Hochstein available for an interview.)

The fighting has driven some 60,000 Israelis from the border area and displaced 90,000 Lebanese. In remarks on July 1, Secretary of State Antony J. Blinken said that Israel had “effectively lost sovereignty” in its north because of Hezbollah’s attacks.

In addition to striking Hezbollah positions over the past several months, Israel has also targeted some of its top commanders. A July 3 drone strike on one commander prompted a retaliatory barrage of more than 100 rockets into Israel. On Tuesday, Hezbollah said an Israeli strike in Syria killed a former bodyguard of the group’s leader, Hassan Nasrallah; a retaliatory attack killed two Israeli parents.

Casualties on both sides have been low relative to the fighting, Mr. Hochstein said at Carnegie. But every day without a diplomatic solution carries risk, he warned, such as an errant missile mistakenly striking a “bus full of children.”

That, he said, could lead to retaliation that triggers all-out conflict “even though both sides probably understand that a fuller or deeper-scale war is in neither side’s interest.”

Mr. Netanyahu has faced growing pressure to restore security so that displaced Israelis can return home safe from Hezbollah rockets, not to mention the now-vivid fear of an Oct. 7-style assault. U.S. officials say that as Israel scales down its campaign against a weakened Hamas in Gaza, it may turn its sights toward a possible war against Hezbollah.

Mr. Hochstein’s mission is to find a diplomatic alternative. U.S. officials say the best hope is a cease-fire in Gaza, which Hezbollah leaders say would cause them to stop their attacks. But even then, Israel would still insist that its northern border be made more secure.

So in addition to trying to restrain the two sides from major escalation, Mr. Hochstein has been negotiating a plan under which Hezbollah would pull back its forces several miles from Israel’s border — possibly in return for U.S. economic aid for southern Lebanon and changes to Israeli military positions.

Israeli officials contend that Hezbollah should be making most, if not all, of the concessions, saying the group has long been in obvious violation of a 2006 U.N. Security Council resolution that ended the last major conflict between Israel and Hezbollah. Hezbollah has flouted the resolution’s effective call that it keep its forces behind the Litani River, some 18 miles north of the Israel-Lebanon border.

While an energy and infrastructure professional like Mr. Hochstein might seem an unlikely mediator for an armed conflict, he is actually revisiting familiar territory. In 2021 and 2022, he helped Israel and Lebanon defuse another potential source of conflict, hammering out a maritime border agreement that determined the rights to lucrative undersea natural gas reserves.

Hezbollah carries strong influence with the country’s Shiite Muslim political leaders, and its assent was required to clinch the maritime deal.

What is more, Mr. Hochstein says, Lebanon has a special place in his heart.

“I fell in love with Lebanon” in 1995, Mr. Hochstein told Mr. Miller, the Carnegie senior fellow. It was his first visit, and he has returned at least once almost every year since, he said. “I’m attracted to the tragedy of Lebanon.”

During his mid-June trip, Mr. Hochstein delivered a particularly sensitive message to Hezbollah. Fearing a miscalculation, he warned its leaders not to assume that the United States could restrain Israel from launching a full-scale attack on the group, according to people familiar with the exchange.

Mr. Hochstein’s background — he is not a trained foreign service officer — has raised some eyebrows among diplomats who note that he is carrying out the sort of sensitive work typically handled by State Department regional experts.

Arab officials and media outlets also remark frequently on Mr. Hochstein’s Jewish heritage and service in the Israeli Defense Forces.

In 2021 Lebanon’s foreign minister, whose country prohibits visits by Israelis, said that he would deal with Mr. Hochstein as a U.S. envoy “and not in his Israeli capacity.” (Mr. Hochstein, born in Israel to American parents, no longer holds Israeli citizenship. He has lived in the United States since the 1990s.)

As evidenced by his trip to Baalbek, analysts say, that has not been a major problem. “He has the trust of these key interlocutors in Beirut and perhaps, one might even infer, of Hezbollah,” said Firas Maksad, a senior fellow at the Middle East Institute.

Because U.S. officials are barred by law from talking to members of terrorist groups, Mr. Hochstein trades messages with Hezbollah via Nabih Berri, the longtime speaker of Lebanon’s Parliament.

Analysts say that the foreign officials Mr. Hochstein speaks with respect his closeness with Mr. Biden.

And as Mr. Biden’s political standing has wavered amid doubts about his viability as the Democratic presidential nominee, Mr. Hochstein has made his own opinion clear.

After The New York Times editorial board on June 28 called on Mr. Biden to leave the race, Senator John Fetterman, Democrat of Pennsylvania, used a profanity in dismissing the editorial on social media.

Mr. Hochstein promptly reposted the message on his personal account.