A day or so after a nuclear weapon was used during wartime, on Hiroshima, a Herald Tribune editorialist considered the “still hardly credible fact” that a “small instrument,” dropped on a “dense population center,” brought about “what must without doubt be the greatest simultaneous slaughter in the whole history of mankind.” That atomic bomb and a second one dropped, on Nagasaki, three days later, killed more than a hundred thousand people, most of them non-combatants. A third was ready to go, but President Harry Truman called it off. Former Vice-President Henry Wallace, then the Commerce Secretary, recalled Truman telling him that “the thought of wiping out another hundred thousand people was too horrible. He didn’t like the idea of killing, as he said, ‘all those kids.’ ” A little more than seven years later, in 1952, the first thermonuclear weapon, almost five hundred times as powerful as the bomb dropped on Hiroshima, was tested near the Enewetak Atoll, in the Pacific Ocean. Truman addressed that test in his final State of the Union message, writing that “the war of the future would be one in which man could extinguish millions of lives at one blow, demolish the great cities of the world, wipe out the cultural achievements of the past—and destroy the very structure of a civilization that has been slowly and painfully built up through hundreds of generations. Such a war is not a possible policy for rational men.” That’s been the rational outlook of American Presidents ever since.

Whether or not Donald Trump, the current American President, is a rational man, not long ago he threatened to commit the greatest act of mass killing in human history, far surpassing the toll from Hiroshima. That came on the morning of September 19th, at the United Nations, an organization founded in the last century in order that, as Trump’s speechwriter put it, “diverse nations could coöperate to protect their sovereignty, preserve their security, and promote their prosperity.” To be precise, what Trump said was that “The United States has great strength and patience, but if it is forced to defend itself or its allies, we will have no choice but to totally destroy North Korea,” after which he added that “Rocket Man”—the name he’s given to the North Korean leader, Kim Jong Un—“is on a suicide mission for himself and for his regime.” Since then, Trump has insulted N.F.L. players (and the league itself) for peaceful protests against racial injustice; supported a losing senatorial candidate in Alabama (and then deleted his tweets expressing that support); and, last weekend, disparaged the mayor of San Juan, Puerto Rico, a proven way to deflect attention from his inattention, in that case to a major natural disaster affecting the lives of more than three million Americans.

Deflection is Trumpian tactic. The U.N. speech, though, has not receded into the deflected past, leaving behind its offhand ambiguity. What did “forced to defend itself or its allies” mean? Rather than say that the United States would strike back if North Korea actually attacked the United States, which never needs to be said, he chose to say something fuzzy, almost unintelligible, on a subject that demands clarity. Perhaps he just wanted to remind the world that a nation with overwhelming nuclear superiority could easily wipe out—“completely destroy”—a nation of twenty-five million people, with a vastly inferior arsenal, and probably do so in just a few minutes. But who needed that sort of reminder?

Trump seems to enjoy provoking Kim, a dangerous and easily provoked young man, uttering the sort of thing, as Evan Osnos wrote on Sunday, “perfectly engineered to trigger Kim’s paranoia and animosity.” As if the phrase “completely destroy” weren’t enough, a few days later, on September 23rd, Trump tweeted—tweeted!—“Just heard Foreign Minister of North Korea speak at U.N. If he echoes thoughts of Little Rocket Man, they won't be around much longer!” What kind of threat was that? It came from the same President who recently said, “With the exception of the late, great Abraham Lincoln, I can be more Presidential than any President that’s ever held this office.” It was therefore not wholly surprising to hear Kim respond by vowing to “surely and definitely tame the mentally deranged U.S. dotard with fire.”

One can only imagine how such name-calling might have affected the most dangerous nuclear moment of the twentieth century—the Cuban Missile Crisis, of October, 1962, when the Soviet Union placed nuclear missiles in Cuba. President John F. Kennedy and Premier Nikita Khrushchev both believed that a vital strategic interest was as stake, but both were nonetheless determined to avoid a war. What if Kennedy, while insisting that the missiles be removed, had taunted Khrushchev as “Fat Nick,” or if Khrushchev, refusing, had called Kennedy a “spoiled punk”? If such language could have nudged the world even a tiny step closer to war, how would history, and morality, have judged that behavior?

The nation, and the world, have learned to shrug off a lot of what Trump says, if only because, soon enough, he says something else, or seems to forget what it was that he said in the first place—which, in the end, erases meaning from much of the record. But it’s the existence of thermonuclear weapons—which today have perhaps three thousand times the explosive power of the Hiroshima bomb—that makes Trump’s irrational outbursts unacceptable, even indecent. How long can rationality go missing before sheer lunacy steps in to take its place?