President Trump. (Jabin Botsford/The Washington Post)

PRESIDENT TRUMP’S abrupt cancellation of a summit with North Korean ruler Kim Jong Un had the same air of hasty, strategy-free improvisation that has characterized his handling of the diplomatic opening all along. Mr. Trump agreed to the summit in March without requiring any action by the North Korean ruler, or even a clear statement of his intentions. He then proceeded to hype the wildly unrealistic possibility that the regime would quickly disarm; he minted a medal to commemorate the upcoming meeting and encouraged talk that he deserved the Nobel Peace Prize.

On Thursday, in apparent response to hyperbolic but entirely unsurprising comments by a North Korean official, Mr. Trump released a loosely worded letter canceling the summit because of the “tremendous anger and open hostility” in the statement. The announcement blindsided the government of South Korea, which had brokered the talks: “We are attempting to make sense of what, precisely, President Trump means,” a spokesman said. Meanwhile, Mr. Trump blurted at a White House appearance that “it’s possible” the summit could still take place on the planned date of June 12, while simultaneously warning that “our military . . . is ready if necessary.”

Never has such chaos attended the public behavior of a U.S. president on a matter of such gravity: Both Mr. Trump and the North Koreans alluded to the possibility of nuclear war. Appearing before Congress, Secretary of State Mike Pompeo was unable to offer an answer when asked what the U.S. strategy would now be. North Korea, meanwhile, had hours earlier made a show of blowing up mountain tunnels it has used to conduct nuclear tests — an action suggesting that until Mr. Trump’s statement, it remained willing to move forward.

White House officials said the North Korean statement that Mr. Trump reacted to was merely the last straw in a series of negative actions. North Korea canceled a planned meeting with South Korea last week and failed to answer U.S. inquiries about summit planning. But Pyongyang was responding, at least in part, to U.S. rhetoric. Mr. Trump and other officials had alluded to the history of Libya, which gave up its nuclear program and later was subjected to a NATO bombing campaign that led to the overthrow and murder of ruler Moammar Gaddafi. North Korea could “end like the Libya model ended if Kim Jong Un doesn’t make a deal,” Vice President Pence said Monday. That led Vice Foreign Minister Choe Son Hui to deride Mr. Pence as “ignorant and stupid” and threaten a “nuclear-to-nuclear showdown” — overheated rhetoric that is familiar to anyone who has studied the North Korean regime.

Mr. Trump has not. On the contrary, until this week he appeared oblivious to increasingly clear indications that Mr. Kim had no intention of quickly surrendering his nuclear arsenal. Rather, North Korea appeared interested only in a multistage process in which denuclearization would be a vague and long-term goal, and the regime would be rewarded for every step forward. That is how previous deals with North Korea have been structured. Such a process carries obvious risks, but the administration should have been willing to carefully explore what Mr. Kim was prepared to do. Instead Mr. Trump has impulsively blown up the process — with potential consequences that he and his administration have not bothered to calculate.