By Max Fisher

New York Times

WASHINGTON >> Donald Trump’s remarks on nuclear weapons have brought him, at times, to a question: Why should he be constrained from ever using them?

The question has, like so many of Trump’s comments, sent shock waves. But nuclear experts say it is shocking not just for the statements themselves, but for the uncomfortable truths they expose, perhaps unwittingly, about nuclear weapons.

In a March interview, Trump asked, “Somebody hits us within ISIS, you wouldn’t fight back with a nuke?” Then on Wednesday, Joe Scarborough, an MSNBC host, said an unidentified foreign policy adviser had told him that, in a briefing, Trump had asked three times, “If we have them, why can’t we use them?” Trump’s campaign has denied this.

Still, the controversy has highlighted a paradox that presidents have grappled with throughout the nuclear age: Nuclear weapons are deployed in great numbers, and at tremendous risk, for the purpose of never being used.

President Dwight D. Eisenhower, though at first a proponent of using nuclear weapons, eventually deemed them too destructive to consider. “You just can’t have this kind of war,” he said in 1957. “There aren’t enough bulldozers to scrape the bodies off the streets.”

Yet the United States and other nuclear powers have maintained and expanded their arsenals, enhancing their ability to launch nuclear strikes even as they have concluded that the logic of such a conflict makes using the weapons unthinkable.

This idea became known as mutually assured destruction, in which countries wield nuclear weapons primarily to deter other nuclear powers. But this deterrent works only if it is credible.

This leads to an odd dynamic: The more willing leaders are to use nuclear weapons, the less likely they will need to do so. Leaders heighten the risk — making the weapons faster, more powerful and harder to stop — so as to minimize it. They make the weapons more usable precisely because they are not.

There is little in Trump’s comments to suggest that he intended to highlight this contradiction, but that is what he did in asking why the U.S. bothers to develop extravagantly expensive weapons it never intends to set off.

“If it seems scary or dangerous that we hinge as much of our security as we do on this contradiction, that’s because it is,” said Kingston Reif, a nuclear expert at the Washington-based Arms Control Association.

Should Trump openly question whether nuclear weapons could be usable, he would not be the first. During the early decades of the Cold War, as the Soviet Union and U.S. gamed out a possible war in Europe, both considered a nuclear chain of events that could be winnable. Eisenhower considered using nuclear weapons in Korea, as did President Richard M. Nixon in Vietnam.

Even today, some analysts say Russian nuclear doctrine allows for a “de-escalatory strike” — a single nuclear detonation meant to halt a conventional conflict. A number of countries, including Pakistan and the U.S., are developing smaller warheads that, because they lower the threshold for use, could be more tempting.

In every case since 1945, at least so far, the terrible risks of nuclear conflict have helped avert its initiation. But, paradoxically, this has only deepened nuclear powers’ belief in the necessity of possessing such warheads, and in developing detailed plans for using them.

Tellingly, though Trump drew outrage when he said in the March interview that he would not rule out using nuclear weapons in Europe, his comments reflected current nuclear doctrine. The U.S. reserves the right use to nuclear weapons under certain conditions, such as retaliation for a nuclear attack, anywhere it deems necessary.

Theresa May, the British prime minister, sparked her own nuclear controversy last month. When a member of Parliament asked her whether she was “personally prepared to authorize a nuclear strike that can kill 100,000 innocent men, women and children,” May answered with a crisp “Yes.”

“The whole point of a deterrent is that our enemies need to know that we would be prepared to use it,” May said.

If it shocks when politicians make such statements, perhaps that is in part because the logic of nuclear weapons can be shocking.

That was the reaction of no less than Dick Cheney, the hawkish former vice president, when he became defense secretary in 1989. That year, Cheney received a series of briefings at U.S. Strategic Command, which oversees nuclear weapons.

At one meeting, meant to introduce him to basic nuclear concepts, he was shown a video displaying plans for retaliatory strikes against Moscow, with a red dot for every nuclear launch. As the video stretched on, Cheney grew visibly aghast.

“Moscow turned slowly into a solid red, covered over and over with ludicrous targets,” a participant at the meeting recounted to Janne E. Nolan, a foreign policy scholar now with George Washington University. “Cheney started squirming around and finally asked one of his military aides why we were doing this kind of thing.”

This spring, when I raised this anecdote in an interview with Defense Secretary Ash Carter, who has long worked on nuclear issues, he said that even he struggled with the implications of nuclear deterrence.

“You never get quite used to how terrible such a situation would be,” he said, but added that the dangers of the nuclear world left no other obvious option.

The need to make this threat credible, by ensuring a capability and willingness to strike, can take on a momentum of its own. In the 1980s, the Soviet Union, fearing that President Ronald Reagan intended to spark a nuclear war, built the ultimate deterrent: the so-called perimeter system, which would trigger an automatic and huge nuclear retaliation. But the Soviet leaders kept their doomsday device secret, defeating its purpose as a deterrent.

Such is the esoteric and at times puzzling logic of nuclear weapons that could lead a presidential candidate, even a very unusual one, to wonder at the point of weapons whose power comes in not being used.

© 2016 The New York Times Company