The conflict in Ukraine has lasted longer than Russian President Vladimir Putin anticipated. Due to unexpected logistical issues, a hostile population, and a tenacious Ukrainian resistance force, the country has been able to repel much of the Russian military and even claim some significant victories.

However, both sides appear to be approaching a stalemate, and a drawn-out war means higher stakes and a greater likelihood that someone will use a nuclear weapon.

The possibility of a nuclear war is never zero. However, since Russia’s invasion of Ukraine, the risk level has risen to levels not seen in decades, and it is now firmly “within the realm of possibility,” according to UN Secretary General António Guterres.

According to experts, the chances of a nuclear war breaking out are impossible to calculate. As long as Western nations avoid a direct military confrontation with Putin over Ukraine, the likelihood of a nuclear conflict is low. They add, however, that as Putin becomes more dissatisfied with how the war effort is progressing, he may become desperate, increasing the odds.

“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 stated clearly why it would use a nuclear weapon, but government officials have yet to rule it out.

“If it is an existential threat to our country, then it can be used in accordance with our concern,” Kremlin spokesman Dmitry Peskov told CNN’s Christine Amanpour on Tuesday.

That statement does not contradict any previous Russian policy on the use of nuclear weapons. Peskov even referred to a document that has been publicly available since June 2020, which emphasises that Russian nuclear weapons should be used only to deter foreign aggressors or if the country believes it is facing an immediate “existential threat.”

Experts, however, point out that Peskov’s vague statement leaves room for interpretation.

“Does it really mean anything?” According to Tom Collina, policy director at the anti-proliferation Ploughshares Fund, a foundation that supports non-proliferation initiatives. “What one person considers to be a ‘existential threat’ is not the same as what another considers to be a ‘existential threat.'”

And in Russia, that one person is President Vladimir Putin, whose authority cannot be overstated.

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

‘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 Ukrainian military’s weakness and ordinary Ukrainians’ readiness to welcome his invading troops with cakes and flowers,” Taras Kuzio, a research fellow at the Henry Jackson Society, recently wrote in an Atlantic Council article.

“Similarly, he appears to have been completely unprepared for the ferocity of the international response or the magnitude of domestic opposition to his invasion,” Kuzio added.

The Ukrainian resistance continues to take the Russian offensive by surprise at every turn. Experts tell Fortune that if Ukrainian forces continue to successfully repel Putin’s offensive, it may force him to go to extreme measures to claim victory.

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

“Similarly, he appears to have been completely unprepared for the ferocity of the international response or the magnitude of domestic opposition to his invasion,” Kuzio added.

The Ukrainian resistance continues to take the Russian offensive by surprise at every turn. Experts tell Fortune that if Ukrainian forces continue to successfully repel Putin’s offensive, it may force him to go to extreme measures to claim victory.

“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.”

Violations of the Geneva Conventions would be considered a crime of war, though it is unclear whether Putin cares about international norms at all.

“We hope that the Russian military will refuse to do so on those grounds, but that is all we can hope for.” Given the Russian military’s behaviour thus far in this war, it’s not something to be expected,” Sagan said.

The Pentagon has already warned that a protracted conflict in Ukraine with no clear winner could lead to more nuclear-weapons threats 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 Pentagon report last week.

According to Sagan, if Putin launched a nuclear strike to end the war on his terms, it would almost certainly result in global condemnation of Russia, as well as the legal status of a wanted war criminal. But it’s unclear whether that matters to him, or how Putin’s calculations may be influenced by developments in Ukraine.

“I don’t think you’ll be able to get those out,” Sagan said. “The course of the war will continue to influence [Putin’s] state of mind.”

Perceived threat from the West

According to Sagan, if Putin launches a nuclear strike to end the war on his terms, Russia will almost certainly face global condemnation, as well as the legal status of a wanted war criminal. But it’s unclear whether that matters to him, or how developments in Ukraine may influence Putin’s calculations.

“I doubt you’ll be able to get those out,” Sagan predicted. “The outcome of the war will continue to have an impact on [Putin’s] mental state.”

NATO officials would prefer to avoid a nuclear war and have warned Putin to back down on his threat. The alliance’s Secretary General, Jens Stoltenberg, recently told reporters that Russia could “never win a nuclear war” and urged the Kremlin to abandon its “dangerous, irresponsible nuclear rhetoric.”

Putin, on the other hand, appears to be bracing for the worst. Since shortly after the invasion, he has kept the country’s nuclear deterrent forces on high alert, ready to respond immediately if necessary.

However, any action by the West, whether genuine or misconstrued, could be interpreted as a threat by Russia’s leader.

“It’s all about how Putin sees things, because he’s in charge of the nuclear arsenal,” Collina explained.

Neither NATO nor the Biden administration have indicated a willingness to send troops directly to Ukraine, instead sending armaments and other supplies to aid the Ukrainian resistance. Despite the human cost of not engaging Russia directly, experts believe that this course of action may be necessary to avoid a nuclear war.

“Having US or foreign military forces in Ukraine, 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.