The possibility of a nuclear strike in Ukraine was considered remote in late February when Russian President Vladimir Putin announced that he had placed his country’s nuclear arsenal on high alert.
But with Russian troops now on the defensive, and conventional warfare unable to check recent Ukrainian battlefield advances, the chances that Putin might resort to nuclear weapons have risen, defense and security specialists say.
“It’s a very serious situation. President Biden is right: This is the worst danger of nuclear weapons being used since the Cuban Missile Crisis,” said Matthew Bunn, co-principal investigator of the Project on Managing the Atom at the Harvard Kennedy School.
The chance that Putin would use a large, cataclysmic nuclear weapon is still considered extremely low. But Bunn estimated the probability at 10 percent to 20 percent that Putin might detonate a smaller one, known as a tactical nuclear weapon, in the Ukraine conflict.
And if such a weapon were used, even as a show of intimidation that resulted in relatively little damage, the pressure on the United States and its NATO allies to retaliate would be intense.
On Thursday, NATO Secretary-General Jens Stoltenberg warned that Putin would be crossing a “very important line” if he were to order the use of nuclear weapons in Ukraine. His remarks came after a meeting of NATO’s secretive Nuclear Planning Group, held among defense ministers in Brussels, and just days before both the military alliance and Russia are due to hold nuclear exercises.
NATO is holding its exercise, dubbed “Steadfast Noon,” next week, and it will involve fighter jets capable of carrying nuclear warheads but that do not have any live bombs. Russia usually holds its own maneuvers around the same time, and NATO is expecting Moscow’s exercise of its nuclear forces sometime this month. Stoltenberg said NATO will “closely monitor” what Russia is up to.
US officials have also been increasingly stern in their warnings over the nuclear risk. Jake Sullivan, the US national security adviser, said last month that “any use of nuclear weapons will be met with catastrophic consequences for Russia.” That warning echoed what John Sullivan, then the US ambassador to Moscow, told the Globe in April.
Russian use of such weapons would trigger a “significant, catastrophic response from the United States and our allies,” said Sullivan, a South Boston native who retired in September.
What that response would look like is highly speculative, but analysts said it probably would include crippling cyberattacks, at the least, and possibly conventional air and missile strikes on troops, equipment, and infrastructure that could place Americans in direct combat against Russians.
A nuclear response by the United States and NATO is not considered likely, analysts said, although that possibility might be placed on the table.
“The people I talk to are not talking about the use of nuclear weapons in response,” said Bunn, who served as a nuclear adviser to President Bill Clinton and added that he is not privy to the Biden administration’s thinking. “They are talking about non-nuclear responses. One possibility is major conventional strikes against Russian forces in Ukraine that would substantially weaken those forces.”
Indeed, French President Emmanuel Macron, in an interview broadcast Wednesday, suggested France would not respond with a nuclear strike. He also warned about the responsibilities of leaders when it comes to nuclear rhetoric and said he has spoken to Putin “several times.”
“We have a (nuclear) doctrine, which is clear,” Macron said. “The dissuasion is working. But then, the less we talk about it, the less we brandish the threat, the more credible we are.”
Russia is believed to have about 2,000 tactical nuclear weapons of various sizes, some of which could target Ukrainian forces on the battlefield. Others could destroy an entire city, for example, or devastate an area as small as 10 blocks. Tactical nuclear weapons have never been used in combat.
In a recent interview with CNN, Biden assailed Putin for even broaching the possibility of nuclear war, although the president added that he did not think Putin would use such a weapon. Still, Biden said, the risks of miscalculation are frighteningly high.
Bunn also does not believe Putin will resort to nuclear weapons, but the uncertainty is unsettling.
“For many things in life, 80 to 90 percent that it won’t happen is satisfactory. But for the use of nuclear weapons for the first time in 77 years, 10 to 20 percent is just intolerable,” Bunn said.
“My concern is that Putin believes he can’t afford to lose [the war], and at the moment, I think he is losing,” he said. “And Ukrainians are highly motivated to do exactly what Putin wants to deter them from doing, which is take back their land.”
Kevin Ryan, a retired Army brigadier general who served as US defense attache in Moscow, put the probability even higher.
“I’d say there’s about a 25 percent chance he might use a nuclear weapon,” said Ryan, a senior fellow at the Belfer Center for Science and International Affairs at the Harvard Kennedy School.
Putin’s angry speech last month, in which he once again threatened to use nuclear weapons, was not what prompted Ryan to believe the nuclear risk had increased. Since then, Russian foreign ministry officials have said that the country is committed to using nuclear weapons only for defensive purposes.
“What has concerned me is that his situation has worsened since February, and he has fewer pathways to escalate,” Ryan said. “He cannot escalate with his ground forces. They are incapable of escalating.”
“It’s not a big step from there to a nuclear weapon,” he added.
Russia launched missile attacks on Kyiv, Ukraine’s capital, and other cities this week, killing 19 people, in retaliation for an explosion that damaged a key bridge to Crimea that Russian forces use for transport. Ryan saw the missile attacks, some of which struck civilian areas, as a sign of desperation.
Ryan envisioned a few nuclear options that Putin could consider: exploding a tactical nuclear bomb over the Black Sea as a show of force to pressure Ukraine to the negotiating table; launching one onto the battlefield in the south and east where Ukrainians have taken back territory that Russia illegally annexed; or even detonating a bomb in a mid-size city.
An aggressive response by the United States “could lead to a big escalation,” Ryan acknowledged. “This is the risk. It’s not a risk-free thing. Nuclear deterrence always had the risk of escalation.”
As the war grinds on, Ryan added, the United States needs to be more direct in its warnings.
“We’ve said we’re thinking asymmetric responses and dire consequences, but these are all fuzzy words and avoid saying out loud, ‘We will put on the table a nuclear weapon,’ ” Ryan said.
One possible deterrent, Ryan added, is telling Russia “that if they use a nuclear weapon on Ukraine, we will give a nuclear weapon to Ukraine, and they can use it as they see fit.”
Despite the rise in nuclear tensions, analysts argue that crossing the nuclear line would impose a cost on Putin and Russia that would far outweigh any temporary benefits gained on the battlefield.
“Whatever international support or tolerance still remains for the war“ — from China and India, for example — “would pretty quickly disappear” if Russia ordered a nuclear strike, said Christopher Miller, professor of international history at The Fletcher School at Tufts University.
“There also is a risk that the military wouldn’t carry it out,” he added. “It’s far from certain that they would do that, and that uncertainty would be pretty dangerous for Putin and his control over the military.”
Material from Globe wire services was used in this report.
Brian MacQuarrie can be reached at email@example.com.