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Mr. Trump tweeted Thursday that “sanctions will remain until an agreement is reached,” which is somewhat reassuring. But like his predecessors, he is giving the Kim regime a substantial reward before it takes verifiable steps toward denuclearization. Even a brief meeting will boost North Korea’s claim to be a nuclear power that must be given respect and recognition. In return, Kim appears to have given nothing other than the promise not to test his weapons in the interim. He can resume those tests at any time.

Mr. Trump can claim credit for putting the diplomatic and sanctions screws on North Korea to a greater extent than any previous President. And it’s possible that pressure may have hurt the North Korean economy enough that Kim chose this moment to change tack. (The Trump critics who claimed he was trying to blow up the world but now say he’s leaping too fast to diplomacy are especially amusing to watch. They wouldn’t give him credit if Kim disarmed entirely.)

But it’s also possible, and perhaps more likely, that Kim is seizing the opportunity to weaken the U.S.-South Korea alliance after the South’s President Moon Jae-in broke ranks with the U.S. and took a softer line on the North. There is no reason to think that North Korea has changed its long-term goals of becoming a recognized nuclear power, expelling U.S. forces from the Korean peninsula and conquering South Korea.

These aims might seem unrealistic, but they are core precepts of the North’s ideology. Kim Jong Un, who took power in 2011 after the death of his father, cannot easily discard them without risking his own legitimacy.

Kim is probably borrowing from his father’s playbook and will use negotiations about nuclear weapons to further those goals. His nuclear and missile scientists will use the pause in testing to consolidate their progress, while North Korean diplomats demand the U.S. sign a formal peace treaty to end the Korean War and withdraw its forces from the South.

That is unacceptable to the U.S. and South Korea because, even without nuclear weapons, the North’s conventional military could do massive damage to Seoul. So will the U.S. offer other sweeteners instead, such as an early relaxation of sanctions? China and Russia, which on Friday expressed support for the summit, will no doubt push the U.S. to allow their trade with the North to resume.

This is the real danger of a summit. It raises expectations so high that the Trump Administration will be tempted to take the advice of career American diplomats who believe the U.S. should accept the North as a nuclear power. They believe the U.S. can then “manage the problem” through a combination of deterrence and appeasement.

The global media circus—and it will be a show for the ages—will also play to Mr. Trump’s self-image as a master negotiator. If he walks away from a summit with nothing, the media will call the exercise a failure and blame him.

All of which makes this summit a walk on the wild side that has more risks of failure for Mr. Trump and the U.S. than for Kim. High-level meetings are appropriate when the North offers specific proposals for denuclearization in lower-level negotiations. That isn’t likely to happen until the country’s economy is on its knees and Kim faces internal threats to his rule. If Mr. Trump goes through with this diplomatic nuclear theater, he’d better be prepared to walk away if Kim does what North Korea has always done before.