Russia digs in: What's the risk of nuclear war? It's not zero.

A major war raging on Russia’s and NATO’s borders. Increasingly bold Western military support. Russian threats of direct retaliation. A mood of siege and desperation in the Kremlin. Growing uncertainty around each side’s redlines.

As Russia and NATO escalate their standoff over Ukraine, nuclear strategists and former U.S. officials warn that there is a remote but growing risk of an unintended slide into direct conflict — even, in some scenarios, a nuclear exchange.

“The prospect of nuclear war,” António Guterres, the United Nations secretary general warned this week, “is now back within the realm of possibility.”

Leaders on both sides emphasize that they consider such a war unthinkable, even as they make preparations and issue declarations for how they might carry it out. But the fear, experts stress, is not a deliberate escalation to war, but a misunderstanding or a provocation gone too far that, as each side scrambles to respond, spirals out of control.

The war in Ukraine heightens these risks to a level not seen since the Cuban Missile Crisis, and in some ways is potentially more dangerous than that, some experts say.

NATO forces, intended as defensive, are massing near Russian borders that, with much of Russia’s military bogged down in Ukraine, are unusually vulnerable. Increasingly paranoid Kremlin leaders, faced with economic devastation and domestic unrest, may believe that a Western plot to remove them is already underway.

Russia has said that it considers the weapons and other increased military aid that Western governments are sending to Ukraine tantamount to war, and has implied that it might strike NATO convoys. Over the weekend, Russian missiles struck a Ukrainian base mere miles from Polish territory.

“Those are the things that make me really concerned about escalation here,” said Ulrich Kühn, a nuclear strategist at the University of Hamburg in Germany.

“The chance for nuclear weapons employment is extremely low. But it’s not zero. It’s real, and it might even increase,” he said. “Those things could happen.”

The Kremlin has turned to nuclear saber-rattling that may not be entirely empty of threat. Russian war planners, obsessed with fears of NATO invasion, have implied in recent policy documents and war games that they may believe that Russia could turn back such a force through a single nuclear strike — a gambit that Soviet-era leaders rejected as unthinkable.

The outcome of such a strike would be impossible to predict. A recent Princeton University simulation, projecting out each side’s war plans and other indicators, estimated that it would be likely to trigger a tit-for-tat exchange that, in escalating to strategic weapons like intercontinental missiles, could kill 34 million people within a few hours.

Alexander Vershbow, NATO’s deputy secretary general from 2012 to 2016, said that Western leaders had concluded that Russian plans to use nuclear weapons in a major crisis were sincere, raising the risk from any accident or misstep that the Kremlin mistook for war.

With Russian forces struggling in a Ukraine conflict that Moscow’s leaders have portrayed as existential, Mr. Vershbow added, “That risk has definitely grown in the last two and a half weeks.”

Murky Redlines

Since at least 2014, when Russia’s annexation of Crimea led to high tension with the West, Moscow has articulated a policy of potentially using nuclear weapons against any threat to “the existence of the state itself.”

Russian statements have subsequently expanded on this in ways that may make the country’s nuclear tripwires easier to inadvertently cross.

In 2017, Moscow published an ambiguously worded doctrine that said it could, in a major conflict, conduct a “demonstration of readiness and determination to employ nonstrategic nuclear weapons,” which some analysts believe could describe a single nuclear launch.

Evgeny Buzhinsky, a retired member of the Russian military’s general staff, has described the aim of such a strike as “to show intention, as a de-escalating factor.” Some versions call for the blast to hit empty territory, others to strike enemy troops.

The next year, Vladimir V. Putin, Russia’s president, said that Russia could use nuclear warheads “within seconds” of an attack onto Russian territory — raising fears that a border skirmish or other incident could, if mistaken as something more, set off a nuclear strike.

A 2020 Russian government paper seemed to expand these conditions further, mentioning the use of drones and other equipment as potentially triggering Russia’s nuclear redlines.

These policies are designed to address a problem that Soviet leaders never faced: a belief that, unlike during the Cold War, NATO would quickly and decisively win a conventional war against Russia.

The result is a reluctant but seemingly real embrace of limited nuclear conflict as manageable, even winnable. Russia is thought to have stockpiled at least 1,000 small, “nonstrategic” warheads in preparation, as well as hypersonic missiles that would zip them across Europe before the West could respond.

But Russian military strategists continue to debate how to calibrate such a strike so as to force back NATO without triggering a wider war, underscoring concerns that threading such a needle may be impossible — and that Moscow could try anyway.

Escalation Risks

“The escalation dynamics of a conflict between the U.S. and Russia could easily spiral into a nuclear exchange,” said Dmitry Gorenburg, an analyst of Russian military policy.

Partly this is because, unlike Cold War proxy battles, Ukraine’s war is raging in the heart of Europe, with NATO and Russian forces massed a relatively short drive away from Moscow and several Western capitals.

Partly it is because of Russia’s lowered nuclear threshold and heightened sense of vulnerability.

But Moscow also seemingly believes that a sort of NATO-Russia conflict has already begun.

Russian strategic doctrine is designed in part around a fear that the West will foment economic and political unrest within Russia as prelude to an invasion.

With Mr. Putin now facing economic devastation and rising protests, “A lot of the pieces of their nightmare are already coming together,” said Samuel Charap, who studies Russian foreign policy at the RAND Corporation.

In these circumstances, Moscow could misconstrue NATO’s troop buildup, or steps of military support for Ukraine, as preparations for just the sort of attack that Russian nuclear policy is designed to meet.

“Between volunteers from NATO countries, all this NATO weaponry, reinforcement of Poland and Romania,” Mr. Charap said, “they might connect dots that we didn’t intend to be connected and decide they need to pre-empt.”

In such a climate, a few mishaps or miscalculations — say, an errant strike or clumsy provocation by one side that sets off a stronger-than-expected retaliation by the other — could escalate, in only a few steps, to the point of triggering Moscow’s fears of an attack.

Mr. Putin has already said that direct Western intervention in the Ukraine war might trigger Russian nuclear retaliation. Now, each uptick in Western support for Ukrainian forces tests those limits.

“Part of our problem is that I’m not sure we have a clear sense of exactly where the lines are,” Dr. Gorenburg said, adding, “This is why we’re seeing all the hemming and hawing back and forth with the question of providing aircraft. There’s just uncertainty as to how the Russians would take that.”

Dr. Kühn, the German analyst, worried that American domestic politics could play a role as well. Should Russia use chemical weapons or commit some other transgression, American leaders could face overwhelming pressure to retaliate beyond what Moscow anticipates.

Many in Washington are already calling for a no-fly zone or other direct intervention, arguing that U.S. warheads would deter Moscow from nuclear retaliation.

But clearing Ukraine’s airspace would likely require striking air bases and anti-air defenses within Russia that also serve to defend Russia’s borders. Analysts caution that such fighting could easily spiral out of control or trigger the Kremlin’s fears of a NATO push to Moscow, leading Mr. Putin to launch a last-resort nuclear strike.

War Games

Christopher S. Chivvis, a former U.S. intelligence official for Europe, recently wrote that “scores of war games carried out by the United States and its allies” all projected that Mr. Putin would launch a single nuclear strike if he faced limited fighting with NATO or major setbacks in Ukraine that he blamed on the West.

The truth is that even Mr. Putin may not know his nuclear redlines for sure. But American fears of Russian nuclear escalation may be dangerous, too.

Any nuclear conflict, however initially limited, carries an escalatory risk that strategists call “use it or lose it.”

Both sides know that rapid nuclear strikes could wipe out their military forces in Europe, even their entire nuclear arsenals, leaving them defenseless.

This means that both sides face an incentive to launch widely before the other can do so first — even if leaders believe that the conflict may have begun in error.

Recent advances in short-range missile technology means that leaders now have as little as a few minutes to decide whether or not to launch, drastically increasing the pressure to launch quickly, widely and with only partial information from the ground.

Late in the Obama administration, two American war simulations imagined an accidental skirmish between NATO and Russia that Moscow met with a single nuclear strike.

In the first, Pentagon leaders proposed a retaliatory nuclear strike to signal resolve. But a civilian White House official, Colin H. Kahl, instead persuaded them to stand down and isolate Moscow diplomatically. Mr. Kahl is now an under secretary at the Pentagon.

But the second simulation ended with American nuclear strikes, underscoring that Washington cannot fully anticipate even its own actions in the event of such a crisis.