‘Button’ It, Mr. President JFK and Reagan had the good sense not to speak flippantly about nuclear weapons.

From the Oval Office address by President John F. Kennedy informing Americans of the presence of Soviet missiles in Cuba, Oct. 22, 1962: “Our policy has been one of patience and restraint, as befits a peaceful and powerful nation which leads a world-wide alliance. We have been determined not to be diverted from our central concerns by mere irritants and fanatics. But now further action is required, and it is under way; and these actions may only be the beginning. We will not prematurely or unnecessarily risk the costs of world-wide nuclear war in which even the fruits of victory would be ashes in our mouth; but neither will we shrink from that risk at any time it must be faced.”

Cuban Missile Crisis

President Kennedy meets with U.S. Army officials during the Cuban Missile Crisis.

From his commencement address at American University, June 10, 1963: “What kind of peace do we seek? Not a Pax Americana enforced on the world by American weapons of war. Not the peace of the grave or the security of the slave. I am talking about genuine peace, the kind of peace that makes life on earth worth living, the kind that enables men and nations to grow and to hope and to build a better life for their children. Not merely peace for Americans but peace for all men and women; not merely peace in our time but peace for all time.

“I speak of peace because of the new face of war. Total war makes no sense in an age when great powers can maintain large and relatively invulnerable nuclear forces and refuse to surrender without resort to those forces. It makes no sense in an age when a single nuclear weapon contains almost 10 times the explosive force delivered by all of the allied air forces in the Second World War. It makes no sense in an age when the deadly poisons produced by a nuclear exchange would be carried by wind and water and soil and seed to the far corners of the globe and to generations yet unborn.”

From the address by President Ronald Reagan after the summit in Reykjavik, Iceland, with Soviet leader Mikhail Gorbachev, Oct. 13, 1986: “I told him I had pledged to the American people that I would not trade away SDI”—the Strategic Defense Initiative. “There was no way I could tell our people that their government would not protect them against nuclear destruction. I went to Reykjavik determined that everything was negotiable except two things: our freedom and our future. I am still optimistic that a way will be found. The door is open, and the opportunity to begin eliminating the nuclear threat is within reach.”

From Reagan’s remarks at the signing, with Mr. Gorbachev, of the Intermediate Range Nuclear Forces Treaty, at the White House, Dec. 8, 1987: “The numbers alone demonstrate the value of this agreement. On the Soviet side, over 1,500 deployed warheads will be removed, and all ground-launched intermediate-range missiles, including the SS-20s, will be destroyed. On our side, our entire complement of Pershing II and ground-launched cruise missiles, with some 400 deployed warheads, will all be destroyed. Additional backup missiles on both sides will also be destroyed. But the importance of this treaty transcends numbers. We have listened to the wisdom in an old Russian maxim. And I’m sure you’re familiar with it, Mr. General Secretary, though my pronunciation may give you difficulty. The maxim is: Dovorey no provorey—trust, but verify.”

Mr. Gorbachev: “You repeat that at every meeting. [Laughter]”

Reagan: “I like it. [Laughter]”

This is how American presidents have always talked about nuclear weapons and the nuclear age—blunt, direct, factual and clear: We never want these weapons used again.

Until now. President Donald Trump’s tweet, 7:49 p.m., Jan. 2, 2018: “North Korean leader Kim Jong Un just stated that the ‘Nuclear Button is on his desk at all times,’ 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!”

We’re not going in the right direction, are we?

Here are the reasons Mr. Trump’s tweet is destructive and dangerous.

Because it is cavalier about a subject that could not be graver. Because the language and venue reflect an immature mind, the grammar and usage a cluttered and undisciplined one. By raising the possibility of nuclear exchange on social media, the president diminishes the taboo against nuclear use. Anything you can joke about on Twitter has lost its negative mystique. Destigmatizing the idea of nuclear use makes it more acceptable, more possible—more likely. Bragging about your arsenal makes it sound as if nuclear weapons are like other weapons, when they’re not.

Using a taunting public tone toward an adversary such as Mr. Kim, who may be mad, heightens the chance of nuclear miscalculation. The president’s tweet is an attempt to get under the skin of a sociopath. Is it a good idea to get under the skin of a sociopath who enjoys shooting missiles?

Blithe carelessness on an issue with such high stakes lowers world respect for American leadership. It undermines our standing as a serious and moral player, which is the only kind of player you would trust, and follow, in a crisis.

The sober and respected Sam Nunn represented Georgia as a Democrat in the U.S. Senate from 1972 to 1997, and is co-chairman of the Nuclear Threat Initiative, a nonprofit trying to prevent the use of weapons of mass destruction. “The danger of nuclear use is greater now than during the Cold War,” he said. The impact of Mr. Trump’s rhetoric? “It increases the risk of blunder.”

There are more nuclear nations, more independent actors, including terrorist groups. “Nuclear material is not fully secured, scientific knowledge of how to make a bomb is increased.” And there is the cyber threat—hacking into weapons systems, supplying false data. “Want a war between India and Pakistan?” Mr. Nunn says. “Simulate a missile attack.” Make it appear missiles are incoming when they’re not.

The risky world becomes riskier. “Add to that the heated rhetoric and name calling, and that increases risk and lays the foundation for a catastrophic blunder.”

You always fear miscalculation and misinterpretations, he says. But the chance of a blundering into disaster is probably greater than the chance of deliberate use.

Mr. Nunn notes we have been lucky that 73 years into the nuclear age there have been no accidental launches, no catastrophic decisions. The nuclear nations have been careful, professional, restrained. But yes, we’ve been lucky.

And should do nothing to press that luck.

Bragging about nuclear arms increases the likelihood of proliferation. “If we’re trying to get countries around the world not to go nuke, then we shouldn’t talk in a way that enhances their importance,” Mr. Nunn says. “There’s a lot of countries out there looking to take their small button and make it into a big button.”

By the way, Reagan’s INF Treaty, that turning point in the history of arms control, remains in force but could unravel due to charges of violations and bad faith. Keeping it up and operating will require work but be heartening for the world.

Focus there. And don’t tweet about it.