The nuclear threats that hang over the world

“A nuclear war cannot be won and must never be fought.” That joint statement was issued at the beginning of this year by China, France, Russia, the UK and the US — the five official nuclear weapons states.

The following month, Russia invaded Ukraine. Ever since, world leaders have been grappling with the threat that a nuclear war might indeed be fought — quite soon.

From the outset, Vladimir Putin has described the conflict as existential for Russia and hinted that he might use nuclear weapons to prevail.

A little more than a week ago, western security officials rushed into their offices over the weekend — alarmed that Moscow’s accusations that Ukraine was poised to use a “dirty bomb” might be a signal that Russia itself was seeking a pretext to go nuclear.

Although that immediate crisis receded, the overall threat that Russia will use a nuclear weapon is still judged to be rising. One scenario discussed in the US government is that a humiliating Russian defeat in the battle for Kherson might persuade Putin to use tactical nuclear weapons against Ukrainian troops in an effort to reverse the tide on the battlefield.

It is sometimes counter-argued that Putin would not use nuclear weapons so close to Russian territory, for fear of contaminating his own country. But senior US officials point out that the smallest tactical nuclear weapons might kill hundreds of people, rather than thousands — and devastate and irradiate just a few square miles.

The US and its allies are focused on preventing Russia from making that fatal step across the nuclear threshold — through a mixture of deterrence and diplomacy. But they are also already thinking hard about the global aftermath of the use of a Russian nuclear weapon. This is unknown territory and the pressure is intense. As one senior US official puts it: “People will be studying how this crisis was handled for decades to come.”

Broadly speaking, there are four main scenarios to consider: nuclear normalisation, nuclear blackmail, avoidance of war and Armageddon.

It is not hard to see how the use of a Russian nuclear weapon could spiral into an all-out nuclear war — leading to what President Biden himself has termed “Armageddon”. Washington has warned that if Moscow were to use a nuclear weapon, there would be a response with “catastrophic” consequences for Russia.

The Americans have not spelt out in public what that response would be. Many commentators think that it would be military, but non-nuclear. General David Petraeus, a former CIA head, has talked of Nato forces attacking Russian troops on the ground in Ukraine with conventional weapons and sinking the Russian Black Sea fleet.

The argument for a western military response is that if Russia got away with using a nuclear weapon — and even succeeded in reversing the course of the war — then the nuclear taboo that has held since 1945 would be smashed.

But direct western military involvement would probably trigger a further Russian response. The west and Russia might then rapidly move up the “escalation ladder”, making the nightmare of all-out nuclear war distinctly possible. As one US official puts it: “I don’t think anyone should be confident that we can control the escalation risks.”

Because the prospect of escalation to Armageddon is so horrific, there is also a real possibility that even the use of a Russian nuclear weapon would not trigger a direct western military response — with the US instead trying to organise the complete economic and diplomatic isolation of Russia. But that would open the door to another disturbing future: “nuclear normalisation”.

Nuclear weapons would have been shown to be tools that can be used in a war of aggression — not just for deterrence. Russia, and even China, might be tempted to cross the nuclear threshold again. And non-nuclear states — such as Japan, South Korea, Germany and a host of others — would rush to acquire nuclear weapons to protect themselves.

Global turmoil would follow the use of a nuclear weapon. Markets would crash and publics might panic across the world, with the possibility of large-scale population movements out of cities.

The fear of such effects is leading to increasing chatter about the need to start peace negotiations with Russia. But western officials are resistant to that move now for fear of a third scenario — successful nuclear blackmail.

If Russia discovers that it can succeed in wars of aggression by simply threatening to use nuclear weapons, another dystopian future beckons. What would stop Moscow from making further nuclear threats, perhaps aimed at eastern Europe? And what conclusions would China or North Korea draw about future conflicts over Taiwan or the Korean peninsula?

The three darkest scenarios — Armageddon, normalisation and successful nuclear blackmail — are all far more possible than they should be. But, collectively, they remain less likely than the fourth possibility — that nuclear war is avoided.

In all previous nuclear crises since 1945, the leaders of great powers have drawn back from the brink. The knowledge that a false move could cause millions of deaths — or even destroy the planet — is enormously sobering. It has kept the world from sliding into nuclear conflict since 1945. It should work again. Probably.

gideon.rachman@ft.com