RUTHFULLY YOURS

Despite the long history of the North’s military threats, only recently has it developed and tested both nuclear bombs and the missiles to deliver them across the Pacific. The Hwasong-15 missile, successfully tested in late 2017, is theoretically able to reach deep into the U.S. mainland. For the current U.S. Presidency, which happens to be Donald Trump’s, that is the hard reality. Something has to be done about this threat, and Mr. Trump has thrown away diplomatic convention to address it.

One can quibble about Mr. Trump’s negotiating style—the over-the-top buttering up of the North Korean dictator, pushing the process to a summit before the principals on either side had arrived at a mutually agreed framework. But in the event, President Trump was willing to walk away without the deal Mr. Kim was offering. It was the right decision.

Based on the details that have emerged, Mr. Kim came to Hanoi with a familiar North Korean offer: He would dismantle all or parts of the Yongbyon nuclear complex in return for lifting economic sanctions.

Afterward, Mr. Trump said the North Koreans wanted all sanctions lifted, and hours later the North’s foreign minister, Ri Yong Ho, insisted they did not ask to lift all sanctions. What he said next was more relevant. The North’s proposal, Mr. Ri said, “was the biggest denuclearization measure we could take at the present stage in relation to the current level of confidence between the DPRK and the United States.”

That is why Mr. Trump walked. Dismantling one nuclear complex in return for lifting major sanctions would have left the bulk of the North’s arsenal and bomb-making facilities unaddressed. The end of sanctions would have restarted revenue flows into North Korea that it could use to improve the bomb and missile-delivery technology it already possesses. The incentive for the North to give up its weapons would have vanished.

Which is to say the summit produced useful information about North Korea’s real intentions. The question is what to do next. Mr. Trump said “we’ll get there” and that talks will continue at lower levels. Perhaps the continuing “maximum pressure” campaign that includes sanctions will cause Mr. Kim to reconsider his lost opportunity to enter the 21st century.

Meantime, the U.S. needs to prepare if Mr. Kim refuses. One concern is that Mr. Trump in his post-summit news conference seemed to take off the table military exercises with South Korea. Mr. Trump went on at length about how expensive these military preparedness drills are, “we don’t get reimbursed,” and so on.

Nuclear proliferation—whether from North Korea, Iran and others in the Middle East or indeed volatile Pakistan—remains the greatest threat the world and the U.S. faces. Having the means to resist it is worth at least $100 million spent on the Korean Peninsula.

Mr. Trump’s willingness to walk is consistent with his posture toward Iran and perhaps it will help in negotiations with China on trade. No doubt he was tempted to accept a deal that he could call a victory while running for re-election. But a continuing nuclear capability in the North would have made that victory as hollow as Barack Obama’s deal with Iran. The difference is that Mr. Trump walked away.