Moon Jae-in jets into Washington on a mission to salvage a rare diplomatic opening between the US and North Korea that is in trouble almost before it begins.
Trump had agreed to meet inscrutable "Supreme Leader" Kim in Singapore on June 12, but the first-ever US-North Korea summit is now in serious doubt, with both sides expressing reservations.
South Korea -- worried about Kim's bellicose weapons testing and Trump's similarly bellicose warnings about a looming war -- was instrumental in convincing the two Cold War foes to sit down and talk.
Moon sent his own national security advisor to the White House in March, carrying an offer of talks and word that North Korea may be willing to abandon nuclear weapons, an enticing prospect.
Trump surprised his guests, his own aides and the world by summarily accepting the meeting, seeing an opportunity to "do a deal" and avoid military confrontation.
Pyongyang is on the verge of marrying nuclear and missile technology allowing it to hit the continental United States with a nuke, a capability Washington sees as wholly unacceptable.
Since then, there has been a landmark series of intra-Korean meetings, two trips to Pyongyang by Mike Pompeo -- first as CIA director then as America's top diplomat -- and three American citizens have been released from the North.
But after several Trumpian victory laps, North Korea's willingness to denuclearize is now in serious doubt.
Earlier this month, North Korea denounced US demands for "unilateral nuclear abandonment" and cancelled at the last minute a high-level meeting with the South in protest over joint military drills between Seoul and Washington.
Trump responded by saying the meeting may or may not take place.
"The president has said, right now it's still on. If that changes you'll find out about it," a noncommittal Treasury Secretary Steve Mnuchin said Monday.
'We'll see what happens'
But Trump also surprised many by offering Kim an upfront security guarantee, allowing him to stay in power, and suggested that Kim's apparent about-face may have been at the behest of Chinese leader Xi Jinping.
"It could very well be that he's influencing Kim Jong Un," Trump said, citing a recent meeting between the pair, their second in a month's time. "We'll see what happens."
Analysts saw North Korea's perceived slow peddling as evidence of what they feared all along, that Pyongyang may have been playing for time -- hoping to ease sanctions and "maximum pressure" or of South Korea overtorquing the prospects of a deal.
"The current episode of tension reflects a wide and dangerous expectation gap between the United States and North Korea," said Eric Gomez of the CATO Institute.
"Denuclearization is not off the table for the North, but it expects the United States to end the so-called 'hostile policy' as a precondition for denuclearization."
It is far from clear what that means concretely, but it could include the forced withdrawal of 30,000 US troops from the Korean peninsula.
With just weeks to go and little clarity on what will be discussed or what happens if talks fail, some Korea watchers predict fireworks during Trump's talks with Moon.
"It increasingly looks like the Moon administration overstated North Korea's willingness to deal. Moon will probably get an earful over that," said Robert Kelly of Pusan National University.
Yonhap news agency quoted a Blue House official as saying Moon would "likely tell President Trump what to expect and what not to expect from Kim."