That President Trump would describe this result as “tremendous progress ” and “very exciting” only makes it more disturbing. Intent on portraying his Korean diplomacy as a success, Mr. Trump appears to be ignoring — or maybe failing to understand — all-too-palpable warning signs.

The administration set the goal of a “complete, verifiable and irreversible denuclearization” of North Korea. But the third Moon-Kim summit meeting, like that between Mr. Kim and Mr. Trump in June, passed without any unambiguous commitment by North Korea to that end. Instead, Mr. Kim spoke of a “Korean Peninsula free of nuclear weapons and nuclear threat,” a formulation that Pyongyang has long defined as including the withdrawal of U.S. forces from South Korea and the removal of the U.S. nuclear umbrella from both South Korea and Japan.

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Mr. Trump’s negotiators have been pushing the North for a show of seriousness about negotiating denuclearization, such as a full disclosure of its arsenal and facilities. Instead, Mr. Kim this week offered the same mostly symbolic measure he outlined to Mr. Trump months ago: the dismantlement of a single engine test site and missile launchpad.

A summit statement said that in exchange for unspecified “reciprocal steps” by the United States, North Korea would also undertake “the permanent dismantlement” of its oldest and best-known nuclear complex, Yongbyon. But the regime has shut down that site before — once even showily blowing up its cooling tower — only to reopen it later. And it is continuing to enrich uranium and build intercontinental missiles at other sites while making plans to deceive the United States about its arsenal, according to U.S. intelligence.

Mr. Moon seems happy to accept and even defend North Korea’s prevarications because his priority is improving relations across the Korean border. He is now pushing Washington to join with the two Koreas in declaring an end to the 1950-53 war by the end of 2018. His plans to open cross-border rail lines and revive a joint industrial zone with North Korea would probably violate U.N. sanctions, unless they are relaxed.

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It could be that Mr. Moon, who is due to meet Mr. Trump at the United Nations next week, will bring word of genuine, substantive steps toward denuclearization that Mr. Kim is willing to take. In that case, reciprocal U.S. action, such as joining in an end-of-war declaration, might be appropriate. In the more likely case that there is no such breakthrough, it will fall to Mr. Trump to resist being stampeded into unilateral and unjustified concessions in order to satisfy Mr. Moon — and his own ego.