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Yet Mr. Moon gave the same assurances about the North’s denuclearization promises a couple of months ago only to have the North tell a different story as the summit approached. The North didn’t answer U.S. phone calls and its emissaries didn’t even show up for a pre-summit meeting in Singapore. Is Mr. Moon selling the same bill of goods now?

A telling moment came Sunday when reporters asked the South Korean leader whether Kim would agree to complete, verifiable and irreversible denuclearization, which are Mr. Trump’s oft-stated terms. According to the Chosun Ilbo newspaper, Mr. Moon dodged the question, saying that the success of the summit would depend on the negotiating details.

Mr. Moon claims to be the mediator between the White House and Pyongyang, but it isn’t clear he’s representing the U.S. position. Instead he is pressing the U.S. to give benefits to North Korea in return for mere steps toward denuclearization. He has adopted the North’s position that the negotiations should agree to “phased and synchronous measures,” meaning the North gets benefits in exchange for incremental steps such as allowing inspectors to visit nuclear sites.

If Mr. Trump falls into this trap, the North will never give up its nuclear arms. Such talks would lead inevitably to lifting sanctions in return for promises and half-measures. The U.S. tried this approach in the 1990s and again in the 2000s. Each time the Kim family regime violated the deal and continued to build its nuclear and missile programs.

Mr. Moon believes he can tame Kim with aid and that the North will never use nuclear weapons against its fellow Koreans. China is on board since such a deal would preserve the North as a buffer state and ease U.S. sanctions on Chinese companies that do business with the North.

For the U.S. and Japan, however, Kim’s nuclear-tipped missiles are an existential threat. He is on the verge of mastering the technology to mount warheads on the intercontinental missile he has successfully tested in the past year. The campaign of United Nations and U.S. sanctions was designed to inflict enough economic pain that North Korea would agree to give up its nuclear weapons completely. Mr. Trump weakened that leverage by agreeing to a summit at Mr. Moon’s behest before the sanctions have had a chance to work.

Mr. Trump spent the weekend saying preparations for the summit are going well, and on Monday the Administration decided not to impose new sanctions. We understand the desire for pre-summit mood music, but the policy question is whether Mr. Trump has adopted Mr. Moon’s appeasement policy as the best that can be achieved. A deal that limits but doesn’t remove Kim’s nuclear threat would have profound consequences for national security as well as the wider cause of nonproliferation. The U.S. would need to bolster its missile defenses extensively—and fast.

Mr. Moon has seen how Mr. Trump can be cajoled with flattery—he floated the Nobel Peace Prize—and the U.S. President clearly wants the showcase of a summit. But summitry is a process to get to a result in the U.S. national interest. That process and result can’t be subcontracted to a Korean President with priorities other than American security.