The war in Ukraine has gone on longer than Russian President Vladimir Putin thought it would. Unexpected logistical issues, a population hostile to invasion, and a dogged Ukrainian resistance force means that the country has been able to fend off much of the Russian military, and even claim some important victories. 

But both sides seem to be approaching a stalemate, and a drawn-out war means bigger stakes and a higher chance that someone may deploy a nuclear weapon.

The risk of a nuclear war is never zero. But since Russia invaded Ukraine, that risk level has crept higher than it’s been in decades, and it’s now firmly “within the realm of possibility,” UN Secretary General António Guterres said in March.

On April 14, William J. Burns, director of the Central Intelligence Agency, said the U.S. government was “very concerned” about the possibility of a nuclear weapon being used. He believes that the state of war and Putin’s numerous setbacks could lead to a “potential desperation” on behalf of the Russian leadership, pushing the Russian army towards using tactical nuclear weapons to carve out something resembling a victory in Ukraine.

Experts told Fortune that the odds of a nuclear war breaking out are impossible to calculate. As long as Western nations do not enter into a direct military engagement with Putin over Ukraine, the likelihood of the conflict going nuclear is small. But they add that as Putin becomes more frustrated with how the war effort develops, he might get desperate, and that means the odds increase.

“The likelihood of Putin actually using nuclear weapons remains low, but the threat is still there,” Shannon Bugos, a senior policy analyst at the Arms Control Association, told Fortune. “None of us really know how the war on Ukraine will develop, and Putin has certainly shown himself to be quite a risk taker.” 

An existential threat that comes down to one man 

Russia has not laid out clear reasons that it would use a nuclear weapon, but government officials have so far failed to rule out doing so. 

“If it is an existential threat for our country, then it can be used in accordance with our concern,” Dmitry Peskov, a Kremlin spokesperson, said in an interview Tuesday with CNN’s Christine Amanpour.

That statement is not a departure from any preexisting Russian policy regarding the use of nuclear weapons. Peskov even referred to a document that has been publicly available since June 2020, which underscores that Russian nuclear weapons should only be used to deter foreign aggressors, or if the country feels it is under an immediate “existential threat.”

But experts point out that Peskov’s vague statement leaves room for any kind of interpretation. 

“It doesn’t mean anything, right?” Tom Collina, director of policy at the anti-proliferation Ploughshares Fund, a foundation that supports non-proliferation initiatives, told Fortune. “One person’s ‘existential threat’ is not another’s.”

And when it comes to Russia, that one person is President Vladimir Putin, whose authority is hard to overstate. 

“So much of this is about one person’s decision-making,” Collina said.

‘A desperate effort to try and end the war’ 

Putin’s hopes of a swift and decisive victory in Ukraine were misguided from the start. 

“He appears to have sincerely believed Kremlin propaganda fairytales about the weakness of the Ukrainian military and the readiness of ordinary Ukrainians to welcome his invading troops with cakes and flowers,” Taras Kuzio, a research fellow at the Henry Jackson Society, recently wrote in an article for the Atlantic Council. 

“Likewise, he seems to have been completely unprepared for the ferocity of the international response or for the scale of domestic opposition to his invasion,” Kuzio added. 

The Ukrainian resistance continues to surprise the Russian offensive at every turn. And experts tell Fortune that if Ukrainian forces continue to successfully resist Putin’s offensive, it might be enough to push him to extreme lengths to claim victory.

“A scenario could be if Putin is losing, and in a desperate effort to try to end the war on favorable terms, he orders an attack against a Ukrainian city,” Scott Sagan, professor of political science at Stanford University and a former consultant to the Pentagon and the Secretary of Defense, told Fortune.

In this scenario, Putin might calculate that one nuclear strike by Russia against Ukraine creates a credible threat that more cities could be targeted, and consequently force President Volodymyr Zelenskyy to step down. If that sounds outrageous, it is worth remembering that it is more or less what happened to Japan at the close of the Second World War.

“It’s pretty much what [the U.S.] did in 1945,” Sagan said, adding that there is one big difference. “For the Russians to drop a bomb on a major city would be illegal today under the Geneva Conventions.”

Violating the Geneva Conventions would constitute a crime of war, even though it is unclear if Putin even cares about international norms anymore. 

“We hope that the Russian military would refuse to do that on those grounds, but that’s only hope. It’s not an expectation given the way the Russian military has behaved thus far in this war,” Sagan said.

The Pentagon has already warned that a protracted war in Ukraine with no clear winner could lead to more threats involving nuclear weapons from Russia. 

“As this war and its consequences slowly weaken Russian conventional strength, Russia likely will increasingly rely on its nuclear deterrent to signal the West and project strength to its internal and external audiences,” Lieutenant General Scott Berrier, director of the Defense Intelligence Agency, wrote in a March Pentagon report.

If Putin were to launch a nuclear strike to end the war on his terms, it would surely incur global repudiation against Russia, and probably earn him the legal status of a wanted war criminal, according to Sagan. But it is impossible to know if that matters to him, and how Putin’s calculations may be affected by developments in Ukraine.

“I don’t think you can break those out,” Sagan said. “[Putin’s] state of mind will continue to be influenced by the course of the war.”

Perceived threat from the West

Ukraine is not a member of North Atlantic Treaty Organization (NATO), which has access to nuclear weapons. But four countries that border Ukraine — Poland, Slovakia, Romania, and Hungary— are. 

Experts that Fortune spoke with do not believe that Western nations or alliances like NATO would be nuclear aggressors against Russia, but if a credible nuclear threat emerged on the border between Ukraine and any of its NATO neighbors, the situation could deteriorate.

“If you have the United States and NATO sending forces into Ukraine, and you get a direct U.S./NATO conflict with Russian forces, then that could spiral into a situation where nuclear war could follow,” Collina said.

NATO officials would rather not see a nuclear war break out, and have warned Putin to back off on the threat. Jens Stoltenberg, Secretary General of the alliance, recently told reporters that Russia could “never win a nuclear war,” and implored the Kremlin to stop its “dangerous, irresponsible nuclear rhetoric.”

But Putin appears to be readying himself for anything. Since shortly after the invasion, he has kept the country’s nuclear deterrent forces on high alert, ready for an immediate response should the need arise.

But any action from the West, whether genuine or misconstrued, could be taken as a threat by the Russian leader. 

“This is all about how Putin perceives things, because he’s the one in control of the nuclear arsenal,” Collina said.

Neither NATO nor the Biden administration have appeared willing to send troops directly to Ukraine, opting instead to send armaments and other provisions to support the Ukrainian resistance. Experts believe that this course of action may be essential to avoiding a nuclear war, despite the human cost of not engaging with Russia directly.

“Having U.S. or foreign military forces in Ukraine and therefore increasing the risk of direct conflict with Russia is the main thing that could lead to a spiral into nuclear war, and the main thing we need to avoid right now,” Collina said.

Updated April 15, 2022: This story was updated with new comments from  William J. Burns, director of the Central Intelligence Agency.

Updated May 4, 2022: This article was updated to reflect the correct name of the North Atlantic Treaty Organization.