So, should Mr. Trump start taking seriously talk about a trip to Oslo? The long litany of aborted efforts in the past, the sheer complexity of any negotiations and the many traps along the way suggest not yet.

Twice before — in 2000 and 2007 — Seoul and Pyongyang made joint peace pledges that crashed on the rocks of North Korean aggression. The vague language of the two Korean leaders’ Panmunjom Declaration on denuclearization falls well short of commitments that Pyongyang made — and then promptly violated — in 2005 and 2012. Despite Mr. Kim’s forward-leaning rhetoric, it is unlikely he is prepared to fully relinquish his nuclear arsenal, which he sees as his best insurance policy against regime change and a ticket to international recognition.

The Kim family long ago perfected the art of the steal: string out talks, wring out economic concessions and walk away from any commitments. Then, when Washington tightens the noose again, ratchet up tensions and repeat. It’s exactly this dynamic President Obama refused to engage.

Even if this time is the exception that breaks the rule and Mr. Kim is serious about denuclearization, the road ahead is long, winding and all uphill.

Mr. Kim enters any talks with Mr. Trump from a position of strength. His charm offensive — ardently embraced by Mr. Moon — is quickly corroding Mr. Trump’s leverage. It takes the credible threat of a pre-emptive strike out of America’s arsenal; the world would not understand Mr. Trump resorting to force when Mr. Kim is talking peace. It is likely to weaken sanctions enforcement, with China and even South Korea looking to ease up on their neighbor. It buys time for Mr. Kim to keep tinkering with his bombs and missiles, albeit without the testing necessary to perfect them. The very fact of meeting with an American president conveys legitimacy to Mr. Kim that his father and grandfather sought but never secured.