WASHINGTON — President Trump lost no time hailing the historic nature of Friday’s meeting between the leaders of North and South Korea. But the gauzy images and vows of peace by Kim Jong-un and his counterpart from the South, Moon Jae-in, have complicated Mr. Trump’s task as he prepares for his own history-making encounter with Mr. Kim.
While the two Korean leaders pledged to rid the heavily armed peninsula of nuclear weapons, they put no timeline on that process, nor did they set out a common definition of what a nuclear-free Korea would look like. Instead, they agreed to pursue a peace treaty this year that would formally end the Korean War after nearly seven decades of hostilities.
The talk of peace is likely to weaken the two levers that Mr. Trump used to pressure Mr. Kim to come to the bargaining table. A resumption of regular diplomatic exchanges between the two Koreas, analysts said, will inevitably erode the crippling economic sanctions against the North, while Mr. Trump will find it hard to threaten military action against a country that is extending an olive branch.
To meet his own definition of success, Mr. Trump will have to persuade Mr. Kim to accept “comprehensive, verifiable and irreversible denuclearization” of North Korea — something that Mr. Kim has shown no willingness to accept in the past, and few believe he will accede to in the future.
“This summit has put even greater expectations, greater hype and greater pressure on Trump,” said Victor D. Cha, a Korea scholar at Georgetown University who was considered by the Trump administration to be ambassador to Seoul. “He hyped this meeting with his tweets, and now the entire focus is going to be on his negotiating prowess.”
“This is a moment of his own making,” Mr. Cha added.
Characteristically, Mr. Trump betrayed no anxiety in recent days as he discussed the challenges of the summit meeting, which is scheduled for late May or early June in a location still to be determined. He took much of the credit for the diplomatic thaw on the Korean Peninsula, and he said he would not commit the mistakes of his predecessors, whom he said had showered the North with money and extracted nothing in return.
“The United States has been played beautifully, like a fiddle, because you had a different kind of a leader,” Mr. Trump said after meeting with Chancellor Angela Merkel of Germany at the White House. “We’re not going to be played, O.K.? We’re going to hopefully make a deal; if we don’t, that’s fine.”
Mr. Trump reiterated that he was prepared to cancel the meeting, or walk out in the middle of it, if his diplomatic efforts were not making any headway. But some of his aides say privately they worry that the president, with an eye on the history books and a flair for the theatrical, is determined to emerge with a victory, even if it falls short of his stated goals.
On Saturday, Mr. Trump said he had a “very good talk” with Mr. Moon. “Things are going well,” he tweeted. “Time and location of meeting with North Korea is being set.” He also said he had briefed Prime Minister Shinzo Abe of Japan, who has watched the rush of diplomacy with some concern.
The price of failure would be high for Mr. Trump. The United States could face a split with its ally South Korea, which is deeply invested in ending its estrangement from the North. Tensions could flare with China, North Korea’s main trading partner, which only grudgingly signed on to the sanctions and would be likely to balk at keeping them in place if Mr. Kim is talking about peace.
Mr. Trump is also moving on other fronts that could undercut his negotiations with Mr. Kim. He appears more likely than ever to rip up the Iran nuclear deal as he faces his next deadline of May 12 to decide whether to reimpose sanctions on Tehran.
Walking away from one nuclear disarmament deal while trying to strike another would be a trick, even for a self-proclaimed dealmaker like Mr. Trump.
There is little question, senior officials and analysts said, that the American-led sanctions, combined with Mr. Trump’s bellicose vows to rain “fire and fury” on North Korea if it threatened the American homeland, helped bring Mr. Kim to the table.
But Mr. Trump is only one of three actors in this drama, and perhaps not the most crucial one. Mr. Moon, a progressive former human rights lawyer, ran for office on a platform of conciliation with the North and has moved aggressively to deliver on that promise. He, not Mr. Trump, has set the pace and terms of the negotiation with the North, though American officials say that Seoul is closely coordinating with Washington.
Mr. Kim, for his part, made a bold bet on diplomacy. His motives for seeking a rapprochement are open to debate. Skeptical analysts said the advancements in North Korea’s intercontinental ballistic missile program — as much as sanctions or threatened military strikes — made the timing right for an overture. Others say he is replaying the cycle of provocation and conciliation pioneered by his father and grandfather.
Whatever his motives, the 34-year-old dictator has proved to be a remarkably adroit player on the world stage. “If anyone gets credit, it’s Kim Jong-un,” said Daniel R. Russel, a former assistant secretary of state for East Asian affairs who is now at the Asia Society. “It’s his show.”
So far, Mr. Kim’s bet has paid off handsomely. Since beginning his overture a month before the Winter Olympics in South Korea, he has been awarded a meeting with President Xi Jinping of China, who had earlier treated him with thinly disguised contempt. He was welcomed by Mr. Moon with a South Korean honor guard. And he is on the verge of something once inconceivable: a meeting with the American president.
Though Mr. Kim made gestures of his own — a pledge not to test bombs or long-range missiles, and an end to the North’s longtime insistence that American troops withdraw from the peninsula — he has not made any tangible concessions on his nuclear weapons. The language in his joint statement with Mr. Moon about denuclearization was both vague and familiar to veterans of past negotiations with North Korea.
“He’s gotten all these meetings with world leaders without making any concessions,” said Jung H. Pak, a former C.I.A. analyst who is a senior fellow at the Brookings Institution’s Center for East Asia Policy Studies. “So far, everything has been no-cost for Kim.”
Given the warmth of the Moon-Kim meeting, few analysts are predicting that Mr. Trump’s meeting with Mr. Kim will be sour. The most likely situation is an encounter that produces more riveting imagery and results in a broad agreement to negotiate disarmament in return for an easing of North Korea’s economic isolation.
On Friday, Mr. Trump said the location of the meeting had been narrowed to two or three sites. Officials had hoped to have already locked down a place, but said the process was more complicated than expected. Singapore and Mongolia have emerged as prime candidates, though an official said a site in South Korea remained a possibility.
The challenge for Mr. Trump will be embarking on a protracted negotiation with Mr. Kim that, if the past is any guide, will quickly bog down in highly technical discussions about inspections of nuclear sites, the dismantling of installations and the removal of nuclear fuel.
Some question whether Mr. Trump’s hawkish new national security team — led by Secretary of State Mike Pompeo and the national security adviser, John R. Bolton — will have the stomach for that. Others note that Mr. Trump may face pressure on the right if it looks as though North Korea is playing for time, as it did during the Bill Clinton and George W. Bush administrations.
With the North seeking to re-establish diplomatic and economic ties to the South, Mr. Trump will find it difficult to play the cards he used during his first year in office. Some analysts said Mr. Kim’s outreach to Mr. Moon amounted to a kind of insurance policy against Mr. Trump.
“It becomes awfully hard for Trump to return to the locked-and-loaded, ‘fire and fury’ phase of the relationship,” said Jeffrey A. Bader, a former Asia adviser to President Barack Obama.
Administration officials acknowledged the risk that Mr. Trump could find himself out of sync with Mr. Moon. They said their job was to remind the president of the proper sequence of negotiations with North Korea: tangible steps toward denuclearization, followed by an easing of sanctions, and then a peace treaty.
As always, though, the wild card is Mr. Trump himself.
“He sees this as a Nixon-in-China moment, and he will want to move quickly, where patience is the order of the day,” said Kurt M. Campbell, a former assistant secretary of state for East Asian affairs.