Since the first day of Putin’s war, the Russian military has conducted two separate but coordinated operations: a ground campaign intended to seize terrain, and an air campaign intended to disrupt Ukrainian logistics and terrorise the population. This is why, as Western nations rallied to support Ukraine with weapons, ammunition and funding, some observers suggested that the West should establish a “no-fly” zone over Ukraine to protect cities and infrastructure from Russian air attacks. Ultimately, the idea was rejected by Western leaders, who understandably erred on the side of caution, not knowing then how many Russian and Ukrainian lives would be at risk or how long the war would last. In hindsight, it was a deadly decision. Russia’s ensuing campaign of air attacks by missiles, drones, and bombs is now a recognised war crime which, according to the UN, kills an average of 500 to 1,000 civilians a month.

Today, everyone has a much better understanding of the outlines of Putin’s war. As Russian military analyst Ruslan Pukhov recently wrote, Russia (and others) initially thought that its “special military operation” would be more of a “special” than “military” operation, and assumed that it could be completed without large-scale military operations or organised military resistance. But now it’s clear that the ground campaign is “doomed”, with “a protracted war” in the style of the Korean or Iran-Iraq wars seemingly inevitable. This means that, while fighting along the front lines will likely stall, Russia’s air campaign could continue unabated for years to come. In fact, we know that it escalated in the first months of 2024.

More importantly, in terms of defence, we also know that, while Russia flies manned aircraft in battles along the front lines, it does not send manned aircraft deep into Ukraine to bomb cities or infrastructure: such attacks early in the war proved too costly for pilots and airframes. Two years later, this means that air-defence systems deployed around major cities deep in Ukrainian territory do not risk killing Russian pilots or troops. Indeed, since the outbreak of the war, the West has already donated a number of air-defence systems to Ukraine, some of which are protecting cities and infrastructure.

But today, what Ukraine lacks as much as the systems are the soldiers to operate them. To help with this, Western countries — which are running short on “spare” air defence systems — could deploy them to Ukraine, but keep them in the hands of the troops that man them.

Of course, this approach carries obvious risks, depending on the nature of Russia’s response. Putin and his leadership circle have frequently reminded the West that, should they intervene more explicitly in the conflict, Russia might resort to nuclear weapons in Ukraine. To that end, Putin has deployed nuclear-capable Iskander surface-to-surface missile units in Crimea and Belarus, the latter of which also contains Russian tactical warheads. Moscow also retains the capability to launch nuclear cruise missiles from deep inside the territory of the Russian Federation. It’s not unreasonable to imagine that Western air-defence units could provoke a nuclear attack in Ukraine — one of the things they are meant to protect against.

For now, the West has hedged against this risk by avoiding any full deployment of troop units into Ukraine. Indeed, when French President Emmanuel Macron said in February that the possibility of Western troops in Ukraine should not be excluded, Putin responded with more nuclear sabre-rattling, saying that, although he didn’t see a reason to use nuclear weapons in Ukraine at this time, “from a military-technical point of view, we are, of course, ready”. And yet, the uncomfortable reality is that we are at an inflection point in this war — one where, in choosing whether to deploy air-defence units to Ukraine’s vulnerable cities, we should be influenced not so much by the hunch that Putin won’t use a nuclear weapon, but by the realisation that he might. And if he does, it would be a travesty if the West had not done all it could to help Ukraine defend itself.

There are several ways that the West could do this. The least provocative scenario, and one sure to avoid killing Russians, would be to take on the mission of protecting some of Ukraine’s key cities in the western half of the country, far from the front lines. All of the five largest cities still under the control of Ukraine — Kyiv, Lviv, Odesa, Dnipro and Kharkiv — are attacked regularly by Russian missiles and drones. Of these, Western nations could take on the mission of helping to defend Kyiv, Lviv and Odesa, which lie on or west of the Dnieper River that divides the country running north to south. Meanwhile, Dnipro and Kharkiv, which lie farther east, closer to the front lines, would remain under the sole protection of Ukrainian air defences.

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What would the new defences look like? The main threats to Kyiv, Lviv and Odesa are ballistic and cruise missiles, as well as long-range drones, which could be countered through a combination of long-range anti-missile systems, such as the US-built Patriot or Russian-built S300/400. Two battalions would provide good coverage for the main parts of a large city, while also being able to intercept and destroy the sorts of missiles that carry tactical nuclear weapons.

Crucially, a dozen countries in Europe use some combination of these defence systems, while America boasts roughly 15 Patriot battalions alone. The US could also volunteer its THAAD installations (Terminal High Altitude Area Defense), which have the added benefit of defending several targets over a wide area, and which are especially designed to protect against ballistic and cruise missiles. Finally, as part of America’s European Phased Adaptive Approach (EPAA), there are fixed-site, American-operated Aegis Ashore interceptors stationed in Deveselu, Romania, soon to be joined by an installation in Redzikowo, Poland. These sites, integrated with US Navy ships, protect against ballistic missile threats from outside the Euro-Atlantic area, and could cover Ukraine too.

But how likely is it that these will be deployed? To date, the EU and Nato continue to rule out sending troops, though some individual members are following Macron’s lead. Estonian Prime Minister Kaja Kallas, who reportedly would like to be the next Secretary General of Nato, recently said “everything” must be on the table to help Ukraine beat Russia, while Lithuania and Poland’s respective foreign ministers voiced their agreement. Such rhetoric shouldn’t be underestimated: since Putin’s invasion, America and the West have looked to the Baltic states to take the lead — whether on the introduction of armoured vehicles, tanks, long-range artillery or aircraft. Only once a new plateau of aid was broached by these countries did the US use the precedent to become a leader in its quantity and quality. The same will be true for any deployment of troops into Ukraine.

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This doesn’t mean that there won’t be impediments. Looking ahead, the presence of Western troops in Ukraine could complicate negotiations when this war nears its end. When, for instance, should they leave? And, more importantly, what will they leave behind? But this insecurity will be worth it if it gives the West a seat at the table when that time comes. We have an interest in the outcome of the war, and having troops on the ground provides leverage that we otherwise would not have. Russia understands this strategy very well. Moscow sent aviators to fight in Korea and air defences and advisors to Vietnam, and it later deployed troops to the Balkans and Syria to steer the outcome of those conflicts.

But more straightforwardly, it’s not too hard to imagine the West deploying air-defence units to protect Ukrainian cities and infrastructure today. We did it before in Iraq, Libya and the Balkans, and, as Macron reminded us, “many of those who are saying ‘never, never’ today are the same ones who said ‘never tanks, never planes, never long-range missiles’ two years ago”. If the West had acted then with air defences, it would have saved thousands of lives. If it acts now, it might save millions.

Putin was quick to brand the strikes a “terrorist” act, while a rattled Yevgeny Prigozhin, head of the Wagner mercenaries, gave war chiefs a dressing-down for their inability to prevent three of eight drones from evading Russian air defences. Yet while this all provided a morale boost for the Ukrainian war effort, the question of retaliation hangs in the air.

Fifteen months into the war, Putin’s bombs have not broken Ukraine. An influx of 300,000 new soldiers over the winter has done little to improve the fighting of Russian units, and the reported deployment of tanks from the Fifties has added fuel to the rumour that Russian munitions are running out. Indeed, Russian military commanders appear to have exhausted their ability to effectively respond to Ukrainian escalation. It is becoming clear, in my view, that the only way Russia can meet escalation with escalation is by introducing nuclear weapons.

Many Western experts say they take the threat of a Russian nuclear strike in Ukraine seriously, but make the mistake of asserting that the odds are low. Last month, for instance, Avril Haines, the US Director of National Intelligence, told a Senate hearing that Putin’s weakened conventional force would make the Russian President more reliant on “asymmetric options” for deterrence, including nuclear capabilities — but he also said it was “very unlikely” that Moscow would do so. Speaking at the same hearing, the director of the Defense Intelligence Agency, Lieutenant General Scott Berrier, also assessed the chances as “unlikely”.

And yet, there is strong evidence that Putin has resolved to use a tactical nuclear weapon in his war in Ukraine. In recent speeches and interviews, he has argued that Russia faces an existential threat — a situation, under Russian policy, that warrants the use of nuclear weapons. He has also reshuffled his military leadership, so that the three generals responsible for the employment of tactical nuclear weapons now command his “special military operation” in Ukraine.

Moreover, while Nato has made it clear that it will not sanction the use of its members’ nuclear weapons to defend Ukraine, Putin already has tactical reasons to deploy them: to save Russian soldiers’ lives, to shorten the war, to destroy Ukrainian forces. He also has strategic reasons: to rejuvenate the deterrent value of his nuclear arsenal and to prove that he is not a bluffer. We must therefore assume he is ready to use them, most likely in response to his faltering military’s inability to sufficiently escalate by conventional means. In other words, the nuclear genie is out of the bottle.

For much of the last 80 years, Russia’s security has rested on two pillars whose relative strength has waxed and waned — its conventional ground forces and its nuclear weapons. The conventional forces have been used to influence, bully and force Russia’s neighbours and adversaries to bend to its will. The nuclear forces were intended to deter the United States and the West from interfering militarily in Russia and its perceived zone of influence. Since the end of the Cold War, however, Russia’s conventional forces have at times struggled with their share of the task. To compensate, Russian leaders have had to rely on their nuclear forces to do both: strategic nuclear weapons to deter the West and tactical nuclear weapons to threaten neighbours.

Today, a single nuclear strike in Ukraine could thwart a Ukrainian counterattack with little loss of Russian lives. For Moscow, this consideration is as much practical as it is moral: last year’s large-scale mobilisation and increase in military units showed that Putin’s army was too small for its task. Nevertheless, Russia has managed to create only a few new battalions because most new personnel and equipment simply replaced losses in existing units. Putin and his military leaders are running out of the people and material needed to achieve his goals.

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At the start of this year, Putin took several public steps to demonstrate that he is not bluffing about using nuclear weapons. In February, he signed a law “suspending” Russia’s participation in New Start, the strategic nuclear arms treaty. This step officially ended joint inspections of American and Russian nuclear weapons sites and released Russia from the obligation to limit its number of strategic nuclear weapons — though Russia promised to do so.

Then, in March, Putin announced that he would station tactical nuclear weapons in Belarus, with a storage facility set to be built as early as July. Since Russia has already deployed nuclear-capable Iskander missile systems there — as well as thousands of troops — this would put nuclear delivery systems and warheads in close proximity to one another, greatly reducing the warning time of their use. Putin also suggested that Belarussian forces would be trained to use the weapons.

The Kremlin has taken these increasingly threatening steps in the belief that Nato and the West — in particular, the United States — is not paying attention to Russian demands on the global stage. In 2018, when Putin unveiled a bevy of new nuclear weapons, he warned: “You will listen to us now!” Except many didn’t: four years later, his invasion of Ukraine was a wake-up call for those who had ignored him.

Despite this, some in Russia undoubtedly fear that the threat of a nuclear strike has begun to ring hollow. And for Putin, whose regime is vulnerable, to threaten a tactical nuclear attack without following through now carries perhaps as much risk as striking. As a result, besides warning the West that he might use a nuclear weapon, the Kremlin has, step by step, prepared the Russian people with reasons why he should use nuclear weapons. Among these justifications, Putin has repeatedly invoked “whataboutist” comparisons to the United States. When announcing plans for deployment of Russian nuclear weapons to Belarus, he said: “The United States has been doing this for decades. They have long… deployed their tactical nuclear weapons on the territory of their allied countries, Nato countries, in Europe, in six states… We are going to do the same thing.” Putin has also repeatedly referenced American nuclear strikes on Hiroshima and Nagasaki and equated American goals then — to save soldiers’ lives and shorten the war — with Russian goals today.

He has, for instance, made clear to the Russian people that Moscow’s red lines for the use of nuclear weapons, spelled out in its official documents, have all been crossed since the invasion. These include the claim that the very survival of Russia is at stake in the current struggle — and at last month’s Victory Day parade, Putin declared that the West’s “goal is to achieve the collapse and destruction of our country”. Another of Russia’s officially designated red lines is attacks “against critical governmental or military sites of the Russian Federation, disruption of which would undermine nuclear forces’ response actions”. Perhaps in light of this, Moscow has alleged that Ukrainian drones have struck strategic nuclear bomber planes inside Russia, and that Ukraine and the US are responsible for drones launched to assassinate Putin. All these claims, the real and the fabricated, are used to establish the pretext for Putin to use nuclear weapons.

In response, a number of Western observers have pointed out that, since we have not seen any movement of nuclear weapons, we have no tangible signs of intent to use them. I disagree. Last autumn, officials in Kyiv reported that Russia was firing “Kh-55 nuclear cruise missiles” with dummy warheads. Observers suggested these missiles — which are designed to carry only a nuclear weapon — were launched to erode Ukrainian air defences by “decoying” them into destroying the Kh-55s rather than missiles with conventional explosives. This claim makes little sense: missiles, even unarmed, would be too valuable for Russia to use as decoys. What does make sense, however, is launching Cold War-era missiles with dummy warheads to test their reliability for use in a real nuclear strike.

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But what will trigger Putin’s decision to launch? Most likely it will be the inability of the Russian military to meet his demands by conventional means. If a Ukrainian offensive threatens, for example, the loss of Crimea, Putin would seek an escalation of the fighting to prevent that loss. If the conventional forces could not successfully respond, a nuclear strike against the Ukrainian forces would be deployed. As he announced last September, on the night he illegally added four Ukrainian provinces to Russia: “If the territorial unity of our country is threatened, in order to protect Russia and our nation, we will unquestionably use all the weapons we have. This is no bluff.”

At home, too, there are push factors that may further embolden Putin. Most urgently, he is under pressure from Russian nationalists, who supported him in his rise to power, but are now vocal in their dissatisfaction. Some, like former FSB officer Igor Girkin, have openly criticised the senior military leadership, even Putin. That criticism may be morphing into opposition, forcing him to consider escalating his war before his conventional forces are ready.

Meanwhile, claims that Putin would be dissuaded from using nuclear weapons by important allies, such as China or India, are not borne out by the war thus far. Although Putin values the support of others, he has not shied away from putting that support at risk to get what he wants.

None of this is to say that we in the West should pressure Ukraine to forgo its goal to liberate all seized territory. But it does mean that we should anticipate a nuclear attack and develop possible responses. As soon as Russia uses a nuclear weapon in Ukraine, the fallout will start to spread. Tens of thousands of Ukrainians will be dead, suffering or dealing with the effects of the explosion. Hundreds of millions of Europeans will be bracing for war. But 7 billion others around the globe will go about their business, alarmed but physically unaffected.

Ultimately, this may prove more dangerous to the international order. The image that many people have of nuclear arms as civilisation-ending weapons will be erased. In its place, such weapons will have been “normalised” and, although tragic, acceptable in war. In this dramatically changed world, the burden is on the West to decide how to respond.

A version of this article first appeared on RussiaMatters.