First Donald Trump called off his secretary of state’s planned trip to North Korea this week. Then Defense Secretary James Mattis suggested on Tuesday that the U.S. might no longer suspend military exercises the North Koreans view as provocative. It’s starting to look like nuclear talks are grinding to a standstill, and a top adviser to South Korea’s president has provided the most detailed description yet of one of the key sticking points: a declaration to end the Korean War.

This is not the same thing as peace. But it is a step in that direction. The Korean War never really ended; the fighting just stopped with a truce in the form of the 1953 Armistice Agreement, which has governed the Korean conflict ever since. What the South Korean government has been advocating for is a political statement that the war is over, which would serve as a kind of bridge between the chronic hostility of the past and a permanent peace in the future.

“The current stalemate comes from the difference between North Korea and the U.S. on which comes first”: the belligerents from the Korean War proclaiming the conflict over, or North Korea disclosing the components of its nuclear-weapons program and permitting international inspectors to access them, said Moon Chung In, a special adviser to President Moon Jae In for foreign affairs and national security. He spoke with The Atlantic after the abrupt cancellation of Mike Pompeo’s visit to Pyongyang.

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Moon, who is also a prominent academic affiliated with Yonsei University in Seoul, characterized the cancellation as “a shock to the South Korean government,” which had been busy preparing for another summit between the leaders of North and South Korea in September. Press reports ahead of the trip suggested Pompeo was on the verge of a breakthrough, in the form of progress on both a declaration to end the war and a North Korean declaration regarding the details of its nuclear arsenal and activities. On Tuesday, CNN, building on reporting by The Washington Post, noted that the immediate cause of the cancellation may have been a letter in which the North Korean government warned the Trump administration that nuclear talks could “fall apart” because of American unwillingness to take “a step forward to sign a peace treaty.”

Now the world’s most ambitious diplomatic effort—in which nothing less than the spread of the most destructive weapons and the prospect of an elusive peace on the Korean peninsula and the future geopolitical order of Northeast Asia are at stake—seems more endangered than at any time since the brief cancellation of the June summit in Singapore between Trump and North Korean leader Kim Jong Un.

The crux of the standoff is this: The United States is insisting that North Korea prove its “sincerity” about denuclearizing by offering a full accounting of its nuclear and missile program, accepting international inspections, and perhaps giving up a certain portion of its nuclear warheads early in negotiations, Moon told me. But North Korea insists progress on peace should come first, as it does in the numbered joint statement Trump and Kim signed in Singapore. The “North Koreans are saying, ‘No, we agreed on a new relationship. And a declaration to end the war in Korea will be the most important token of [the] new relationship,’” said Moon.

South Korea’s president, who first proposed the end-of-war declaration, has so far not managed to persuade the United States and North Korea to pursue both steps simultaneously rather than getting stuck on the sequencing. The Trump administration is concerned about weakening its bargaining position by agreeing prematurely to what some officials consider an “irreversible” end-of-war declaration, while the North Korean government believes it is America’s “turn to reciprocate” after the North’s closure of a nuclear-test site and partial dismantlement of a missile-engine-test site, Moon told me. “If Kim Jong Un accepts those American terms then he would be losing his face to the North Korean military,” he said.

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The South Korean government already has its own vision for an end-of-war declaration—one Moon Jae In has presented to both the North Koreans and the Americans. It has four components, according to Moon Chung In.

The first is political leaders taking the “symbolic” step of formally declaring the end of the Korean War, more than 60 years after the heads of U.S.-led United Nations forces and North Korean and Chinese forces signed the armistice (South Korea adhered to the truce but refused to sign it). There was a time when this declaration might have involved only the leaders of the United States and the two Koreas. “Our president was ready to pay a visit to Singapore during the U.S.–North Korea summit, but that didn’t work out,” Moon said. “Pyongyang and Washington were preoccupied with the success of [their] bilateral summit. They decided to hold the summit [for only] one day.” But Moon now also expected the leader of China, which in addition to being a party to the original armistice is also North Korea’s most powerful ally and has “the utmost leverage” with the North, to take part in the declaration.

With hopes dashed for an end-of-war declaration during the Trump-Kim summit and later in July during the 65th anniversary of the Armistice Agreement, the Moon Jae In administration is now setting its sights on the United Nations General Assembly in late September in New York. “Wouldn’t it be wonderful if President Trump, President Xi Jinping, President Moon Jae In, and Chairman Kim Jong Un meet together at the United Nations [and] adopt the declaration to end the Korean War?” Moon Chung In asked. “That would be a really epochal event for peace and denuclearization in Korea.”

The second dimension of an end-of-war declaration would be a pronouncement on the “elimination of hostile relations” between North and South Korea and between North Korea and the U.S., which could entail “anything short of diplomatic normalization,” according to Moon. This might encompass, for example, the continued suspension of U.S.–South Korean military exercises and training and a halt to America’s deployment of nuclear-capable military assets like bombers near the peninsula; the United States and North Korea exchanging liaison offices in Pyongyang and Washington (an inter-Korean liaison office is already in the works); and the U.S. government issuing statements about not posing a “conventional and nuclear threat to North Korea” and potentially easing sanctions if the North makes substantial progress on denuclearization.

The third aspect of the declaration would be stipulating that the Armistice Agreement and the arrangements that stem from it—such as the Military Demarcation Line that separates the Koreas and the United Nations Command that seeks to deter North Korea from the South Korean side of the border—would temporarily remain in place until the parties negotiate a more comprehensive, legally binding peace treaty to replace the Armistice. And the fourth component would be linking the completion of a peace treaty and normalization of diplomatic relations among the parties, along with a broader “peace regime on the Korean peninsula,” to the denuclearization of North Korea.

“Initially all the parties showed a positive response” to the South Korean government’s proposal for an end-of-war declaration, Moon said. Donald Trump enthusiastically endorsed the idea. At their first summit in Panmunjom in April, Moon Jae In and Kim Jong Un vowed to pursue the declaration within the year—an ambitious timeline the South Korean government deliberately sought not just “to make a transition from the Armistice Agreement to some form of peace treaty,” but also as a way to “expedite the process of North Korea’s denuclearization.”

As South Korea’s ambassador to the United States, Cho Yoon Je, told me earlier this summer, against the backdrop of reports that North Korea is largely continuing to develop its nuclear program even as it negotiates over the program’s future, an end-of-war declaration might afford North Korean leaders the interim security assurances he suspected they need to move toward full denuclearization and a final peace treaty.

But some in the United States and South Korea warn of the unintended consequences of an end-of-war declaration, no matter how logical and innocuous it might seem at first glance. Cheon Seong Whun, a national-security official in the conservative administration of former South Korean President Park Geun Hye, told me recently that such a declaration would increase pressure for a peace treaty even if North Korea hasn’t sufficiently rolled back its nuclear program. This, in turn, could dissolve the U.S.–South Korea military alliance and leave a jilted South Korea exposed to the aggression of a still-nuclear-armed North Korea, he cautioned. (Many South Korean conservatives are critical of the liberal Moon administration’s North Korea policies.)

The end-of-war declaration “is a perfect formula for North Korea to claim that our military exercises, our sanctions, our criticism of human rights are all breaking this opportunity for peace,” weakening all three forms of U.S. pressure on North Korea, said Michael Green, a former Asia adviser to President George W. Bush, during a panel last week at the Brookings Institution. Moon Jae In’s “idea was that you have these two wheels [of peace and denuclearization] connected by this axis and you’re just going to keep moving,” the North Korea expert Jung Pak observed at the same event. “There was always the fear that the peace wheel was going to move too fast and that denuclearization was not going to move … And I think that’s what’s happening.”

Such consequences help explain why, as The Washington Post reported this week, Pompeo is confronting resistance to the end-of-war declaration within the Trump administration. Moon acknowledged the risks but argued they’re manageable. “Yes, in the process of adopting a declaration to end the war North Korea could demand a withdrawal of American troops [from South Korea], but neither the United States nor South Korea would accept it,” he said. “Nothing”—not even declaring an end to the Korean War—“is irreversible other than the death of a human being.”

“I don’t know,” he admitted, when asked how to break the impasse, “but we will continue to tell Washington and Pyongyang ‘you gotta engage in talks.’” And what would the South Korean government do if North Korea refuses to denuclearize to the Trump administration’s satisfaction and the United States pulls out of those talks? “We’ll consult with the United States. We are allies. No doubt about it,” Moon told me. “But that is Plan B. We never talk about Plan B.”