Nuclear weapons have, since their inception, been a paradox: the mightiest of weapons that we dare never use. The nature of this paradox was best exemplified by the Cold War doctrine of MAD—mutual assured destruction. If both sides have vast arsenals of nuclear weapons, then both sides can destroy each other. Thus, using nuclear weapons becomes unthinkable because it would pointlessly lead only to an apocalyptic stalemate, and so no side would be crazy enough to try. But this theory doesn’t cover all the possibilities of nuclear conflict.

What if only one side in a conflict has nuclear weapons? What’s stopping the nuclear-armed side from using its nuclear weapons in that scenario? What if a nuclear first strike can knock out the nuclear arsenal of the other side? Is the use of nuclear weapons unthinkable in that scenario? What if one side launches only a limited nuclear attack, such as using battlefield tactical nuclear weapons? How should the other side retaliate in that scenario?

What if a madman, zealot or fanatic obtained nuclear weapons, someone divorced from rational calculation or uninterested in preserving human life? How does the fear of nuclear retaliation deter such a person?

But in the end, the Cold War concluded without anyone using nuclear weapons, and it has now been 77 years since they were last used in combat on Hiroshima and Nagasaki. But the paradoxes of nuclear weapons haven’t gone away. In my recent book, Big wars: why do they happen and when will the next one be? ^[1], I explore the most likely scenarios in which nuclear weapons might conceivably be used in the future.

The most likely scenario is if a nuclear-armed nation were facing a potential military defeat in a conventional war against a non-nuclear foe. In such a scenario it might be all too tempting for the nuclear-armed side to try to solve its military problem with nuclear weapons. The non-nuclear victim of a nuclear attack could not retaliate, and there would be serious doubts about whether others would retaliate on its behalf given the fear that they might then be next.

Informing my choice is the fact that it has been this precise scenario that has seen the world come closest to nuclear weapons being used in the past 77 years. The United States contemplated using nuclear weapons during the Korean War in 1950 when its conventional forces were being overrun by a Chinese assault. In 1954, President Dwight Eisenhower contemplated using nuclear weapons to end the communist Viet Minh siege of the French base of Dien Bien Phu in Indochina. In 1968, President Lyndon Johnson contemplated using nuclear weapons to end the North Vietnamese siege of the US base at Khe Sanh in South Vietnam. In 1973, Israeli Prime Minister Golda Meir contemplated a nuclear response to the attack on her country by Egypt and Syria during the Yom Kippur War. All of these examples involved a nuclear-armed nation losing a conventional war to a non-nuclear-armed foe.

If someone did make the callous decision to resort to nuclear weapons in such a scenario, they would be lumped with international opprobrium to be sure, but in the desperate circumstances of potentially losing a war, that might not restrain the unscrupulous. Which brings us to the current war unfolding in Ukraine.

Details are sketchy, but what seems clear is that the Russian ground attack has faced serious opposition, far tougher than many expected, and there seems a risk of Russian forces getting bogged down. In the face of harsh sanctions, the costs of a drawn-out war will mount, and an increasingly unhinged Russian President Vladimir Putin may get desperate. He has already warned of consequences ‘such as you have never seen in your entire history’ for those who interfere with his schemes in Ukraine, a thinly veiled threat to use nuclear weapons, followed days later by his decision to put Russia’s nuclear forces on high alert.

Many in the West fear Putin may start a nuclear war. I fear he will use nuclear weapons, but not against the West. If Russia’s conventional forces continue to struggle in Ukraine, the use of tactical nuclear weapons in that country seems more plausible. Ukraine couldn’t retaliate since it gave up its nuclear weapons after the collapse of the Soviet Union. Would the US or NATO seriously risk a global apocalypse by retaliating on its behalf? Putin, if desperate, may calculate that they won’t.

Many are already claiming that the war in Ukraine is a turning point in history, with major global ramifications for defence spending, alliance strategies, and energy and climate change policies. But that historical turning point will become more precipitous and jagged if this conflict sees the 77-year taboo on the use of nuclear weapons come to an end. But that long taboo is part of the reason Putin might contemplate using them. If nuclear weapons are abhorrent and unthinkable to the West, then in the eyes of the unscrupulous the West is unlikely to use them, so the risk of retaliation is small, especially if the West is not directly attacked.

This is the paradox of nuclear weapons: the less we want to use them, the harder it is to deter others from using them.