Opinion | We can’t unmake nuclear weapons. But we can decide never to use them.

That detonation and the similar U.S. bombing of Nagasaki three days later miraculously remain the only instances when atomic weapons have been used in anger. Between 110,000 and 210,000 Japanese civilians were killed in the two blasts, according to the Bulletin of the Atomic Scientists, which keeps a “doomsday clock” estimating how close the world is to the “midnight” of cataclysmic nuclear war.

Before the explosions in Japan, there were precisely two armed-and-ready nuclear weapons in the world. Today, according to the Federation of American Scientists — founded after World War II as the Federation of Atomic Scientists by experts alarmed at the destructive power of the atomic bomb — there are roughly 12,500 nuclear warheads in the stockpiles of nine nuclear nations. And many of those weapons are thousands of times more powerful than the rudimentary devices used against Japan in 1945.

Kishida decided to hold this year’s meeting of G-7 leaders in Hiroshima to focus renewed attention on the existential question of nuclear disarmament. The issue deserves a more serious response than what Kishida is likely to get: high-minded speeches, and perhaps establishment of some kind of mid-level “working group” whose studies and proposals go nowhere.

With Russian President Vladimir Putin threatening to use “tactical” nuclear weapons in his bloody attempt to conquer Ukraine, and with China and North Korea both developing new missiles capable of delivering nuclear warheads, the response from the G-7’s nuclear powers — the United States, France and Britain — is not going to be a genuine push for disarmament. Instead, it will be to spend whatever is necessary to ensure that allied nuclear capabilities match or exceed those of our adversaries.

Three decades after the end of the Cold War, we are still trapped inside the conceptual framework of mutual assured destruction. No leader dares to use nuclear weapons because the response would be a devastating retaliatory nuclear strike. Nuclear powers go to great lengths to avoid direct armed conflict with one another, given the risk that a conventional war might somehow go nuclear. It is even possible to argue that because of this deterrent effect, nuclear weapons are not a bane to humanity, but a blessing.

One problem with mutual assured destruction is that it doesn’t work unless all the nations that possess nuclear weapons are truly prepared to use them. Another problem is that the system incentivizes nations engaged in ideological or territorial conflict with the West to develop nuclear weapons of their own: Once you have the bomb, nobody is going to mess with you.

Iraq failed to make a nuclear bomb, and Saddam Hussein was deposed. North Korea succeeded, and Kim Jong Un remains comfortably in power. It is only logical that Iran’s mullahs would race to follow Kim’s example — rather than suffer Hussein’s fate.

Consider India and Pakistan. Perhaps the fact that both are nuclear powers keeps their occasional border clashes from escalating. But what if a skirmish somehow gets out of hand, perhaps fueled by the rising militancy of Hindu nationalists? A nuclear exchange on the subcontinent could kill tens or even hundreds of millions.

If you want some idea of what nuclear war would be like, read “Hiroshima,” written by New Yorker journalist John Hersey one year after the bombing. He interviewed survivors and reconstructed a shockingly vivid account of the blast and its aftermath. A Methodist minister, Rev. Kiyoshi Tanimoto, described the detonation as “a sheet of sun” and saw houses two miles from the epicenter flattened. Survivors had horrible burns from the explosion and the massive firestorm that followed. The lucky died quickly; the less fortunate died in agony from radiation sickness in the days and weeks that followed.

The detonation of a single modern nuclear warhead over a large city anywhere in the world would be exponentially worse.

We cannot unlearn how to make nuclear weapons; that’s not how human knowledge works. But we can, as a civilization, decide that nuclear holocaust will not be our fate. We had the ingenuity to unlock the cataclysmic power of the atom. We must somehow find the wisdom to forswear its use.