A South Korean soldier walks past a television screen showing pictures of President Trump and North Korean leader Kim Jong Un. (Jung Yeon-Je/AFP/Getty Images)

PRESIDENT TRUMP is right to pursue negotiations with North Korea, but his sudden decision to accept an unprecedented summit with Kim Jong Un compounds the already high probability of failure. Mr. Trump seemingly granted the dictator a prize his regime has sought for decades — a face-to-face meeting with the U.S. president, on apparently equal terms — in exchange for none of the actions the White House previously said were necessary, including verifiable steps toward nuclear disarmament.

A Trump-Kim summit would transfix the attention of the world, which is probably one reason it is appealing for Mr. Trump. Arguably, it would give him a chance to assess a ruler who, until his planned encounter with South Korea’s president next month, will never have met a foreign leader. But if Mr. Trump believes that North Korea is ready to denuclearize, he is almost certainly wrong. Which raises the question: Does the president have a strategy for using the meeting to U.S. advantage?

Unfortunately, the evidence suggests that the Korea summit, like Mr. Trump’s recent decision on trade tariffs, was embraced by the president without much thought. Even Secretary of State Rex Tillerson appeared taken by surprise; he had just told reporters that negotiations might be a long way off. On Friday the White House added to the confusion by suggesting that Mr. Trump would insist on preliminary actions by North Korea after all. The chaos is not surprising, considering that the president lacks expert staff. There is no ambassador to South Korea, and the special envoy for North Korean issues recently retired.

If it holds, the agreement on talks will have a short-term benefit because North Korea has agreed to suspend what had been a constant stream of nuclear and missile tests while negotiations continue. Administration officials contend that Pyongyang has been moved by the pressure of sanctions, which Mr. Trump has succeeded in intensifying in the past year. While that may be true, the risk is that the regime will follow its well-established pattern: capture the world’s attention with provocative acts; then agree to negotiations to extract economic and political concessions; then break any commitments it has made.

The challenge for Mr. Trump is avoiding another repeat of that cycle. That means setting an achievable goal: A realistic one might be a long-term extension of the freezing of missile and nuclear tests in exchange for limited U.S. concessions. Unfortunately, by agreeing to a summit, Mr. Trump has already handed over one of the largest potential trade-offs free of charge. Mr. Kim will use the event to portray his murderous regime as a legitimate nuclear power able to parlay with the United States on equal terms. He will no doubt demand a U.S. withdrawal from South Korea and a formal peace treaty in exchange for denuclearization, in the hope that Mr. Trump, unlike every previous U.S. president, will swallow those terms.

Given the president’s decision, the administration’s best course would be to use the coming weeks to hold preliminary talks with the Kim regime that make clear U.S. expectations and place the summit on firmer ground. If it becomes evident that the North is unwilling to commit to a freeze on its nuclear and missile activities, or will make excessive demands in exchange, Mr. Trump can step back. What he should not do is walk blindly into an encounter with a dictator who, we can be sure, will be well-prepared to take advantage of this president’s well-known weaknesses — starting with his penchant for impulsive decisions.