PRESIDENT TRUMP and the governments of North Korea and South Korea are combining to create sky-high expectations for a diplomatic breakthrough on the North’s nuclear program. In the case of the Koreans, that’s evidently the result of careful and clever tactics. In Mr. Trump’s case, it looks more like empty and self-aggrandizing bluster. The result is that the president’s bet on an unprecedented summit with the North Korean leader is looking riskier than ever.

South Korean President Moon Jae-in and North Korea’s Kim Jong Un, who will stage their own summit on the inter-Korean border on Friday, share an interest in creating a sense of diplomatic momentum. Mr. Moon, a committed dove, was alarmed by Mr. Trump’s bellicose rhetoric toward North Korea and by reports the White House was considering a preemptive strike. South Korea would like to draw the United States into a multi-phase negotiation with North Korea that would, at least, minimize the threat of war for some time — even if previous such processes have failed.

Mr. Kim just promised his people he would propel the country’s decrepit economy into an “upward spiral,” something that would require the lifting of U.N. economic sanctions. By announcing the suspension of nuclear and intercontinental missile tests last week, and letting it be known that he would not ask for the withdrawal of U.S. troops from South Korea, the 34-year-old dictator is probably seeking to create the expectation of compensatory American concessions when he and Mr. Trump meet, without taking any of the “verifiable” and “irreversible” steps toward denuclearization the Trump administration has been demanding.

Mr. Trump seems all too happy to play along. “Big progress!” he tweeted in response to the freeze announcement Friday. On Sunday he added: “Wow, we haven’t given up anything & they have agreed to denuclearization . . . !” On Tuesday his enthusiasm reached an unseemly climax: “Kim Jong Un ,” he said of a dictator who has brutally murdered would-be rivals and maintains the world’s largest gulag, “really has been very open and I think very honorable from everything we’re seeing.”

In fact, Mr. Kim’s public statements have omitted any mention of denuclearization, and though the South Koreans maintain his regime is open to it, most experts believe there is little or no chance Pyongyang will give up its nukes anytime soon. That raises the question of whether Mr. Trump is prepared for a negotiation on something less than the full and quick elimination of North Korea’s arsenal. Would he accept the multistage deal Mr. Moon is promoting? What interim concessions would he be willing to make in order to preserve Mr. Kim’s easily reversed testing freeze?

If Mr. Trump has carefully thought through these questions, there is little sign of it. Instead, he promotes the prospect of a major breakthrough, while promising to walk out if he doesn’t get it. That could be a recipe for failure. Mr. Trump and his aides should work out what sort of outcome from the summit is both realistic and in the American interest, and then design a careful strategy to obtain it. That, after all, is what the Koreans are doing.

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