President Trump’s abrupt decision to cancel the planned summit meeting with Kim Jong-un is not surprising, given the decades of volatile relations between the United States and North Korea.

It’s also not necessarily bad news — if it means that the Trump administration will now take the time to do the preparation needed to make such a high-stakes meeting successful. But it will prove deeply regrettable, and ultimately dangerous, if it winds up meaning that the two hotheaded leaders sulk off and resume the schoolyard taunts that they exchanged over the past 16 months. That would only make conflagration more likely.

In yet another sign of Mr. Trump’s carelessness even with his allies, he failed to consult with South Korea, which had helped broker the meeting, before calling it off. “We are trying to figure out what President Trump’s intention is and the exact meaning of it,” a South Korean spokesman said.

Even under American administrations that brought more time, expertise and discipline to the task, negotiations and engagement with North Korea have proceeded in fits and starts, ultimately ending in failure and greater mistrust. We hope the president can now find a way to continue the dialogue with Pyongyang and reschedule the meeting, as he suggested he was willing to do in his breakup letter to Mr. Kim on Thursday. Late Thursday, a North Korean vice foreign minister, Kim Kye-gwan, said his government is set “to resolving problems at any time in any way.”