On the morning of Aug. 18, 1976, two American soldiers set off to trim a poplar tree in the Korean demilitarized zone (DMZ). The tree was obscuring the line of sight between U.N. and North Korean guard towers on the narrow strip of land that has separated the peninsula’s communist North from its capitalist South since an armistice effectively ended the 1950 — 53 Korean War. Both sides had approved the pruning, but North Korea sent soldiers to order the work to stop. Captain Arthur Bonifas and First Lieutenant Mark Barrett refused, and were promptly hacked to death with their own axes.

General Richard G. Stilwell, then commander of the U.N. Forces in South Korea, ordered the tree completely cut down as a symbolic act of resolve. Among the troops sent to help fell the tree was a young South Korean soldier named Moon Jae-in. Tensions were dangerously high, he says today. “If the North had tried to interfere, it could easily have triggered war.”

War is again a possibility on the Korean Peninsula–and Moon may soon be once again at the front line. The former human-rights lawyer, 64, is the clear front runner for President in the upcoming May 9 election, called after the impeachment of President Park Geun-hye over a corruption scandal. South Korea has many problems, including the Asia-Pacific’s worst income inequality, rising youth unemployment and anemic growth. But the campaign has turned on how best to deal with North Korean Supreme Leader Kim Jong Un, who is locked in a standoff with new U.S. President Donald Trump over his country’s nuclear program. Kim unveiled a new generation of ballistic missiles at a glittering parade on April 15, and conducted the latest in a series of tests on April 29, just hours before a U.S. Navy strike group–an “armada,” as Trump put it–was due to arrive at the Korean Peninsula. China’s Foreign Minister Wang Yi has warned that “conflict could break out at any moment.”

So South Korea’s next President will inherit a deepening crisis with an irascible dictator on one side and a geopolitical neophyte on the other. But Moon, the center-left Democratic Party candidate who narrowly lost the presidency in 2012, believes it is his destiny to bring the two Koreas closer together after seven decades apart. “The North and South were one people sharing one language and one culture for about 5,000 years,” he says. “Ultimately, we should reunite.”

As a son of refugees from the North, Moon is determined to go his own way about it–tackling the Kim regime not by aggression but by measured engagement. The current cycle of antagonism helps no one, he says, least of all the long-suffering population of the Hermit Kingdom. “My father fled from the North, hating communism. I myself hate the communist North Korean system. That doesn’t mean I should let the people in the North suffer under an oppressive regime.”

Moon was born in the shadow of war. His parents fled the North aboard a U.N. supply ship in December 1950 alongside thousands of other refugees. Moon was born on South Korea’s Geoje Island just over two years later. The postwar South had neither the heavy industry nor the fertile farmland of the then more prosperous North. “Poverty dictated my childhood,” he says now. “But there were benefits as well: I became independent, more mature than my peers, and I realized that money is not the most important thing in life.”

By the time Moon entered adulthood, money had begun flowing into the South. The country experienced rapid economic growth from the 1960s on, driven by export-led tech, automotive and shipbuilding booms. Moon grew to prominence as a pro-democracy student activist, passing the state bar exam in 1980. Following a distinguished legal career, he was invited to join the administration of former President Roh Moo Hyun. Today, the economy he hopes to lead is the world’s 12th largest by GDP. In contrast, the North stagnated under a Soviet-style planned economy. Now, the nation of 25 million is one of the world’s poorest.

Moon is aware that reunification would entail a colossal financial burden for the South. That’s why the first step in bringing the countries together must be economic cooperation, he says. He wants to allow South Korean firms access to cheap North Korean labor, and renew cultural exchanges across the DMZ. “Economic integration will not only benefit the North,” he says, “but also will give the South a new growth engine, which will revive the South Korean economy.”

But gradual reunification presents an existential as well as an economic challenge. Today’s DMZ does not just separate two unequal states–it divides the kitschy consumerism of a freewheeling South and the festering paranoia of a Stalinist North. Few pairs of states are so close yet so far apart–and even fewer have a rogue dictator, heavily armed, so intent on standing in the breach. The main challenge for any leader of the South will always be how to deal with Kim Jong Un.

Relations between North and South aren’t merely bad; there are no relations. The last summit between Pyongyang and Seoul took place a decade ago, and even at the DMZ there has been no official dialogue since 2013 — when U.N. forces want to communicate with their North Korean counterparts, they use a megaphone to bellow across the gap. For Moon, this is unacceptable. “Even if Kim is an irrational leader, we have to accept the reality that he rules North Korea,” he says. “So we have to talk with him.”

There are some signs Kim has begun to relax his grip. Although dissent is still ruthlessly quashed, he has permitted a free market to take root, and the much maligned state distribution bureaus — once responsible for doling out all provisions — are shuttered. New buildings spring up constantly in Pyongyang, where flatscreen TVs and karaoke machines are common, and locals now talk of a “rush hour.” In his New Year speech in 2015, Kim Jong Un even said he was open to talks with the South. The sticking point, as ever, is the nuclear issue. Aware of his fragile leverage, Kim has repeatedly said that the country’s nuclear weapons are “nonnegotiable.” For Moon, talks would be worthwhile only with “a guarantee that there would be visible results such as freezing or dismantlement of [the] nuclear weapons program.”

Moon has seen these kinds of negotiations in action before and believes they can work again. As chief of staff to Roh, he helped engineer the South Korean President’s historic summit with Kim’s father Kim Jong Il in 2007, and the six-party denuclearization talks between North and South Korea, the U.S., China, Russia and Japan, which ran from 2003 to 2009. A satellite launch by Pyongyang ended the talks, and critics say the $4.5 billion of aid funneled to the regime during the “sunshine policy” of engagement actually accelerated the weapons program. Moon, however, points to the Sept. 19, 2005, Joint Declaration — encompassing full dismantlement of North Korean nuclear weapons, a peace treaty and even normalized relations with the U.S. — as evidence the sunshine policy was better than the following decade of isolation and censure. “The North even blew up the cooling tower of its nuclear reactor,” he says. “The same step-by-step approach is still workable.”

Given Trump’s stated disdain for the nuclear deal the U.S. helped fashion with Iran, it’s hard to imagine he would be eager to pursue a similar agreement with the Kim regime, which has a track record of noncompliance. But Moon says he and Trump already agree that the Obama Administration’s approach of “strategic patience” with North Korea was a failure. Surely the U.S. President could be persuaded to take a different tack, he says. “I recall him once saying that he can talk with Kim Jong Un over a hamburger.” Trump, he adds, is above all a pragmatist. “In that sense, I believe we will be able to share more ideas, talk better and reach agreements without difficulty.” Indeed, on May 1, Trump told Bloomberg that he “would be honored” to meet Kim.

There are few safe alternatives. Trump is currently pressuring China, responsible for 90% of North Korean trade, to turn the screws on Pyongyang and take steps against Chinese businesses and banks doing deals with North Korea. “China has great influence over North Korea,” he has said. Perhaps, but the relationship today is steeped in mistrust. Beijing has signed up to unprecedented U.N. sanctions, banning imports of coal for the rest of the year. There is room for Beijing to do more: suspending the 500,000 tons of crude oil it sends to North Korea annually, for example, was what brought Kim Jong Il to the six-party talks in 2003.

However, China has its limits. If the Kim regime collapsed, a massive influx of refugees would certainly make their way into the People’s Republic. South Korea is also home to 28,500 U.S. troops, and reunification might put them right on China’s border. So Kim knows China would never squeeze enough to foment its collapse. “It’s like trying to bluff at poker when the other players can see your cards,” says John Park, director of the Korea Working Group at Harvard Kennedy School.

Military action by the U.S. also remains a possibility, but most experts think it’s unlikely. Aside from possible North Korean retaliation, any strike would certainly shred the U.S.’s Asian security alliance and push the region closer to China. “How would the U.S. or anyone else be better off?” asks Daniel Pinkston, an East Asia expert at Troy University in Yongsan, South Korea. “It’s just insane.”

All of which leaves room for Moon’s push for engagement to succeed. Moon’s chief rival in the May 9 election, Ahn Cheol-soo, a self-made tech multimillionaire, favors a more militaristic approach to bringing the North to the negotiating table. This includes accepting the U.S. Terminal High Altitude Area Defense (THAAD), an antimissile defense system, which Beijing deems an affront. Moon, who was 21 points ahead of Ahn in an April 29 poll, is more cautious on THAAD, saying its deployment should be examined by the next administration.

But both candidates are united in their insistence that South Korea cannot be sidelined when Washington deals with the North, not least as its 50 million citizens stand to be among the first victims of any military conflict. And although younger South Koreans feel little affinity with the North, older generations are eager for the reunification Moon so desires. “My mother is the only one [of her family] who fled to the South,” Moon says. “[She] is 90 years old. Her younger sister is still in the North alive. My mother’s last wish is to see her again.”

It’s a wish that resonates with countless ordinary Koreans — on both sides of the battle lines — who want peace to triumph over war.

–With reporting by ZOHER ABDOOLCARIM and STEPHEN KIM/SEOUL

This appears in the May 15, 2017 issue of TIME.

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Write to Charlie Campbell at charlie.campbell@time.com.