Over the weekend, Russian President Vladimir Putin gave orders to his nation's nuclear forces. On Monday, the U.S. said it would not respond with changes to its own nuclear posture.
"At this time we see no reason to change our own alert levels," White House press secretary Jen Psaki told reporters on Monday.
Some experts, though, are worried about the possibility of nuclear escalation. Here's why.
The exact meaning of Putin's order remains unclear
In a brief clip, Putin is shown speaking to two stony-faced generals about the country's nuclear forces.
"He basically said, 'Because of all these hostile or aggressive statements and aggressive policies, we should start this special mode of combat duty of our deterrent forces,'" says Pavel Podvig, a senior researcher at the United Nations Institute for Disarmament Research in Geneva.
It's unclear what a "special mode of combat duty" actually is. One possibility, says Podvig, is that the order activated the nation's nuclear command and control system.
"Normally, in peacetime, the command and control system is configured in a way that makes the transmission of an actual command very much impossible," he says. "It's like you could press the button, but then nothing happens, because the button is not connected to anything."
Putin's order may have meant he wanted the button activated.
Then again, it may not.
Podvig says a follow-up statement from the Russian Ministry of Defense implied it may just mean upping the staffing at facilities that support nuclear weapons. It could be "they just added a few more people to the crews," Podvig says.
Russia has a lot of nuclear weapons at the ready
Russia has more nuclear weapons than any other nation on Earth, according to Hans Kristensen, director of the nuclear information project at the Federation of American Scientists.
"We estimate that they have about 4,500 or so nuclear warheads in their military stockpile," he says.
For now, Russia's largest nuclear weapons — aboard its submarines, bombers and intercontinental ballistic missiles — appear to be at their usual level of alert, Kristensen says. But the nation's stockpile also includes nearly 2,000 so-called tactical nuclear weapons, which are kept in storage facilities throughout Russia.
"They were developed for the purpose of fighting a limited regional battle. Sort of a nuclear war in a very small area," says Kristensen.
The U.S. has about 100 nuclear bombs stationed across Europe that could be used for tactical nuclear warfare.
The Kremlin's battlefield weapons can be launched on the same short-range missiles Russia is currently using to bombard Ukraine, such as its Iskander ballistic missile.
Right now, there's no indication that the battlefield nukes have been pulled out of storage.
Russia says it would use nuclear weapons only as a last resort, but some are skeptical
Russia officially says it would use nuclear weapons only if the nation's very survival was at risk. But not everyone thinks its nuclear rules are so clear-cut.
"A lot of people have questioned whether the bar for Russian nuclear use is as high as its official statements say," says Olga Oliker with the International Crisis Group.
In 2018, the Pentagon's nuclear posture review warned that Russia might use a battlefield nuke to "'de-escalate' a conflict on terms favorable to Russia." In other words, Russia might detonate a smaller weapon to get its opponents to back off.
That statement was somewhat controversial among arms control experts at the time. Oliker believes such action would only possibly happen in a direct war with NATO forces.
In the current conflict with Ukraine, "I think it's very unlikely that Moscow is just going to lob a nuclear weapon at something," she says. "Obviously it's been a week when a lot of people's assumptions have been challenged, but I'll cling to this one for a while."
The risk of miscalculation is higher than it's been in years
Putin's latest statements may amount to little more than nuclear saber-rattling, says Jeffrey Lewis, a senior scholar at the Middlebury Institute of International Studies at Monterey.
"Putin has had a pretty bad-news week," he says. "The Ukrainian army is fighting back, which he didn't expect. The Russian army is performing dreadfully. They are indiscriminately shelling civilian areas. Those things all make him look weak, and the best way to push those headlines down a little bit is a nuclear threat."
But Lewis says there is still plenty of nuclear risk. Putin has already miscalculated in his invasion of Ukraine.
"What would happen if the Russian warning system had a false alarm in the middle of a crisis like this?" he asks. "Would Putin know it was a false alarm? Or would he jump to the wrong conclusion?"
Even if the short-range battlefield nuclear weapons are still on the shelf, thousands of Russian and American long-range missiles are ready to launch in just minutes. That threat hangs over everything as the conflict in Ukraine drags on.
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