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The prospect of an armed conflict on the Korean Peninsula is almost too horrific to conjure. But Pentagon planners, including Defense Secretary Jim Mattis, have had to imagine what would happen if the long-simmering standoff over North Korea’s nuclear weapons and ballistic-missile programs led to a military confrontation.

Such a conflict would be “probably the worst kind of fighting in most people’s lifetimes,” Mattis said on “Face the Nation” in May. “The bottom line is it would be a catastrophic war if this turns into combat if we’re not able to solve this situation through diplomatic means.”

It’s a foreign policy given that cooler heads need to prevail. Unfortunately, but predictably, President Donald Trump acted in the opposite manner on Tuesday. Responding to North Korean leader Kim Jong Un, who in a New Year’s Day speech said he had a “nuclear button” on his desk, Trump tweeted: “Will someone from his depleted and food starved regime please inform him that I too have a Nuclear Button but it is a much bigger & more powerful one than his, and my Button works!”

At a time when steady leadership is needed, Trump offered the impulsiveness of a teenager. While North Koreans need to account for Kim, those close to Trump, including Mattis and the other generals with significant sway in the White House — as well as congressional Republicans continually complicit in placating the president — should confront him on how reckless it is to bait a nuclear-armed adversary.

Cooler heads are also needed in crafting the U.S. response to rare outreach from Pyongyang to Seoul. As part of his address, Kim signaled that the North might send an Olympic delegation to the upcoming Winter Olympics in South Korea, and on Wednesday a dormant border hot line was reopened. That restores direct communication, which could result in direct diplomacy to ease military tensions between the two nations.

There should be no naïvete about North Korea: It is a repressive regime brutally suppressing its people in what are likely crimes against humanity. It’s also a rogue, nuclear-armed state that intends to keep its arsenal. The multinational coalition arrayed against the North is right to strive for denuclearization of the peninsula, and it should be clear-eyed about a potential attempt to weaken the U.S.-South Korean alliance.

But if this dispute is ever going to be solved diplomatically, it will require negotiation and, yes, compromise, and that requires dialogue. The hot line may facilitate such diplomacy and avoid a hot war that could indiscriminately kill civilians and military personnel on both sides of the border.

The U.S. needs a strategic response that increases the chance of a negotiated settlement while keeping the hard-earned international unity on sanctions intact.

Doing so won’t be easy. And it won’t be done with tweets. Instead, it will take a steady, sturdy approach with allies and adversary alike.