It’s been more than 75 years since nuclear bombs were detonated over the Japanese cities of Hiroshima and Nagasaki in August 1945, bringing World War II to an end with Japan’s unconditional surrender. Since then, somehow, the world has avoided further use of nuclear weapons in anger, even during grave crises such as the 1962 Cuban missile crisis, the 1973 Yom Kippur War and the 1983 Able Archer incident.

In 2022 the world faces a new nuclear threat, with the risk that Russian President Vladimir Putin’s invasion of Ukraine could turn into a wider war between NATO and Moscow that escalates past the nuclear threshold or, alternatively, Russia’s use of a tactical nuclear weapon ^[1] in Ukraine. CIA Director William Burns said ^[2] on 14 April: ‘Given the potential desperation of President Putin and the Russian leadership, given the setbacks that they’ve faced so far, militarily, none of us can take lightly the threat posed by a potential resort to tactical nuclear weapons or low-yield nuclear weapons.’

A Russian defeat at the conventional military level would increase the likelihood of Putin going nuclear, perhaps as part of a strategy ^[3] of ‘escalate to de-escalate’ in which a low-yield tactical nuclear weapon is detonated in Ukraine. Such a move would either seek to turn the tide of battle or serve as a warning shot to Kyiv and NATO to accept Russia’s terms for ending the war.

It’s also possible that Russia could decide to escalate at a conventional level by extending its attacks beyond Ukraine. Russian Foreign Minister Sergei Lavrov has accused ^[4] NATO of engaging in a proxy war and said that weapons shipments are legitimate targets. And Russia is already making implied threats of extending the war ^[5] to the disputed Transnistria region of Moldova ^[6]. That would dramatically increase the threat to Romania, a NATO member, and destabilise the Moldovan state, many of whose residents are ethnically Romanian.

Perhaps most worryingly, Putin recently doubled down on the nuclear rhetoric with an implicit threat ^[7]:

If someone intends to intervene in the ongoing events from the outside and create strategic threats for Russia that are unacceptable to us, they should know that our retaliatory strikes will be lightning-fast. We have all the tools for this, things no one else can boast of having now. And we will not boast—we will use them if necessary. And I want everyone to know that.

With the West expanding its assistance to Ukraine, the possibility that Putin could interpret it as intervention generates another pathway to escalation.

It’s not clear how NATO would respond to the use of a low-yield nuclear weapon in Ukraine—or, for that matter, large-scale use of chemical weapons against Ukrainian targets. The chemical weapons scenario is perhaps more likely, given that norms of non-use of chemical weapons have already been eroded by Syria’s large-scale use of a range of them against its own people in 2014. Use of such weapons by Russia might simply attract intensified sanctions and political condemnation. Tactical nuclear use would be a different matter altogether.

Use of a nuclear weapon—even a low-yield tactical weapon—would represent a fundamental shift in global security. It would shatter the norm of non-use of nuclear weapons, and absent an effective response by NATO, would usher in a new era in which states would perceive such weapons as credible options for warfighting, not just for deterrence.

Other nuclear-armed states might move to prioritise low-yield tactical nuclear weapons, and non-nuclear states that had nuclear ambitions, such as Iran, might decide that participating in non-proliferation and arms control is no longer a priority. Negotiations on restoring the nuclear deal with Iran could become a casualty of nuclear escalation in Ukraine and North Korea is already well into developing ^[8] a range of new tactical nuclear forces.

Of course, not responding—or responding weakly, such as with intensified economic sanctions and political condemnation—isn’t the only option open to NATO in the event Russia uses a tactical nuclear weapon in Ukraine. Direct military intervention at a conventional level, to strike at Russian nuclear-capable delivery systems, would be one option; another would be deployment of NATO forces on the ground to directly support Ukrainian forces in battle.

But any direct military intervention by NATO, even below the nuclear threshold, would almost inevitably lead to a wider NATO–Russia war, and with it, the near certainty of nuclear escalation. It’s that spectre of nuclear war—as opposed to a single detonation—that constrains NATO’s responses, even in the face of Russian atrocities in Bucha and Kramatorsk. In particular, the prospect of such a war escalating to strategic nuclear exchanges and devastating the planet will be in the minds of NATO decisionmakers.

So, there’s a risk now emerging that in the face of military defeat at the conventional level, Russia will use nuclear weapons and plunge the world into a new and uncertain future. It’s a future in which low-yield nuclear weapons become usable in conflicts, certainly in terms of implicit and explicit coercive threats against military intervention—as China might do in a Taiwan crisis. In the worst case, a different perception of the operational utility of low-yield tactical nuclear weapons emerges in comparison to strategic nuclear forces. The nuclear genie is out of the bottle, and the question is whether it can ever be put back in.

A Russian use of low-yield nuclear weapons that quickly leads to Kyiv’s acceptance of terms dictated by Moscow would be the worst of all outcomes in Ukraine—at least apart from a broader war leading to global thermonuclear war. Moscow would change the international security order for the worse, dramatically escalating the threat of a war with NATO and worsening the continent’s security outlook, while fundamentally shifting the perception of the utility of nuclear weapons. A key norm in the rules-based order would collapse, along with non-proliferation. Instead, Western liberal democracies would have to reconcile with states that saw nuclear weapons as highly desirable capabilities for deterrence, for coercion and for use.

In the Indo-Pacific, we’d need to consider the prospect that China might alter or abandon its no-first-use nuclear policy and place greater emphasis on developing tactical and substrategic nuclear forces for coercion and possible use in a future Taiwan crisis.

Russia’s explicit and implicit nuclear posturing sets a dangerous precedent of threats to coerce, in which any response may lead to uncontrolled escalation to nuclear war. In effect, Moscow has demonstrated a failure of Western deterrence below the threshold of strategic nuclear war and, at the same time, has achieved escalation dominance at the tactical nuclear level. It’s a lesson that won’t be lost on other states.