“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 Mr. 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 United States 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 United States, 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 Mr. 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 United States reserves the right to use 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,” Ms. 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,” Mrs. May said.