Washington Post columnist David Ignatius deconstructs President Donald Trump's fiery rhetoric on North Korea. Don't give Trump too much credit for disrupting international relations. (Adriana Usero,Kate Woodsome/The Washington Post)

If you never have to grow up, then all the world’s a playground. This is the case for both President Trump and Kim Jong Un, notable examples of nepotism run amok. They stand on either side of the playground yelling epithets at each other and vowing some awful stuff. We know, of course, who is in the right here, but both leaders need a timeout.

Kim threatens the United States with nuclear weapons. Trump counters by threatening “fire and fury.” The North Koreans had earlier said they would unleash “a sea of fire” on the mainland United States, which is a horrific phrase, but not more so than the one Trump later used. It has a grave pedigree — echoes of Harry S. Truman’s warning to Japan after the atomic bombing of Hiroshima. He said that if the Japanese “do not now accept our terms they may expect a rain of ruin from the air, the like of which has never been seen on this Earth.”

Three days later, the United States dropped an atomic bomb on Nagasaki. It was the last ever used in a war.

The precedents here are chilling. And it is no comfort that Trump’s remark was reportedly made extemporaneously, yet another blurt from an unthinking, and still unthinkable, commander in chief. This is not a schoolyard. Voices need to be lowered, accommodations need to be made. The diplomats, so scorned by Trump, have to take over.

It is impossible to read or hear anything about the North Korean leader without the word “unpredictable” attached like a caboose. But why? Kim is anything but. He has been on a steady march to becoming a nuclear power all the time he has ruled North Korea. This is what he wants — maybe more than anything. More than improving his people’s standard of living. More than gaining the benefits of rapprochement with South Korea. Even more than assuring the Chinese that he will not visit a nuclear holocaust on their region.

If anything, it is Trump who’s unpredictable. He is a man who casually breaks his word, whose disloyalty to subordinates is now manifest, who lies incorrigibly and who reacts emotionally and adolescently to challenges or rebuffs. He treats world leaders as he did his Republican primary opponents, and the pettiness and ugliness of that was a departure for American politics — never, as they say, exactly bean-bag to begin with. Is his “fire and fury” threat credible, or is it something that he merely thought sounded good — a verbal Mexican wall?

The horror is that we may now rely on the common sense and steadfastness of Kim to avoid an unimaginable outcome. Both sides here have much to lose, but Kim could lose all — his regime and his very life. North Korea is not likely to survive a war. This time, there will be no armistice at the 38th parallel. A second Korean War might well be fought on Gen. Douglas MacArthur’s terms. Nuclear weapons could be used. China could come in, as it did once before.

North Korea is a menace — Trump is right about that. But he is wrong if he thinks inflammatory rhetoric is going to dissuade Kim from his path. He needs to let the new sanctions take effect. He needs to unleash his diplomats and seek a back-channel because the United States and North Korea have no formal diplomatic relations. A miscalculation on either side could lead to war. Someone needs to come up with a deal. Donald, isn’t that what you do?