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How likely is a Russian nuclear strike in Ukraine?

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Russian leader Vladimir Putin has raised the specter of using a nuclear weapon in his war with Ukraine. Putin has done this before, though he was more explicit in a speech last Friday, and he says he's not bluffing.

The U.S., he said, set a "precedent" when it dropped two atomic bombs on Japan in 1945 at the end of World War II. So what is the likelihood that the Russian leader might use of such weapons?

"We have a 77-year tradition, some call it a taboo, of non-use of nuclear weapons. Russia is threatening that," said Matthew Bunn a professor at the Harvard Kennedy School. He's spent his career studying nuclear weapons, including a stint in the 1990s as a White House adviser when President Bill Clinton was in office. "We need to do everything we can to maintain that tradition of not using nuclear weapons in combat."

Here are a number of key questions surrounding Russia possible use of nuclear weapons.

Q. Why is Vladimir Putin raising the possibility of a nuclear attack now?

Ukraine's military keeps regaining ground in the east and the south of the country, which Russia had seized earlier in the war. With these battlefield setbacks, Putin is facing pressure from the pro-war camp in Russia. Many of these hardliners say Russia needs to unleash the full force of its military in Ukraine.

This has likely contributed to Putin's recent escalation, which includes the mobilization of 300,000 additional troops, annexing Ukrainian territory, and the nuclear saber rattling.

Q. So is Putin just trying to appease some of his critics, or might he actually use a nuclear weapon?

The Russian leader is probably the only person who can answer this question with authority. Most nuclear experts say the likelihood of Russia actually using a nuclear weapon is still relatively low, but given Putin's current predicament, and his public statements, the threat is seen as increasing.

Bunn said his best estimate is that there's a 10 percent to 20 percent likelihood that Russia might use a nuke. While that's a pretty low probability for most things in life, when it comes to nuclear weapons, it is "intolerably high," Bunn said.

Q. How could a nuclear strike help Russia on the battlefield?

It's not clear that such an attack would give Russia a major military boost.

Russia has an estimated 2,000 small, low-yield, tactical nuclear weapons that could be used against specific targets — like a concentration of troops, a military base, perhaps a port or an air field.

However, Bunn notes that "one problem with using nuclear weapons, especially if you detonate them on the ground where they suck up a bunch of dirt and rock into the air, is the place where you use them ends up being uninhabitable because radioactive fallout."

Russia can do just as much damage, if not more, with a relentless barrage of conventional weapons. The Russian military has already devastated a number of Ukrainian cities with conventional firepower.

Bunn said if Russia uses a nuclear weapons, the intention may be mainly to intimidate Ukraine and the West.

"I think the biggest factor in the use of nuclear weapons is the fear they provoke," Bunn said. "Putin might hope that he could coerce the Ukrainians into accepting his terms, that he could coerce the West into backing off from supporting Ukraine."

Q. How are Ukraine and the U.S. reacting at this point?

A senior U.S. defense official told reporters Monday that the the U.S. is keeping close watch, and hasn't seen any Russian moves that would compel the U.S. to change its own nuclear posture.

Ukraine hasn't said much about Putin's most recent threat, but the country has a first-hand understanding of nuclear dangers following the 1986 accident at the Chernobyl nuclear power plant in Ukraine, which ultimately claimed thousands of lives.

While Ukraine has multiple nuclear power plants for civilian use, the country does not have nuclear weapons. Ukraine inherited a large nuclear arsenal when the Soviet Union broke up in 1991. But Ukraine gave up its nuclear weapons under a 1994 agreement under which Russia pledged to respect Ukraine's borders.

Q. What might the U.S. do if Russia actually set off a nuclear weapon?

President Biden's administration has warned of a powerful, though unspecified U.S. response. The U.S. president has sought to avoid a direct military confrontation with Russia, but a Russian nuclear strike could change the calculus.

According to Bunn, such an attack could lead the U.S. to hit Russian military targets, though he thinks it unlikely the U.S. would respond with nuclear weapons.

"People have talked about things like conventional attacks on Russian forces in Ukraine," he said. "Things that would be extremely unpleasant for the Russians and make the cost of using nuclear weapons higher than the plausible benefits."

Q. Beyond the battlefield, what kind of international response could Russia expect?

Russia is so far coping with international sanctions. But punitive measures would surely increase in the wake of a nuclear attack, and Russia's remaining partners might seek to distance themselves from Moscow.

The response of two countries, China and India, would be particularly important. China's ties with Russia were expanding, but Beijing is growing uncomfortable with the trajectory of the war. India has officially remained neutral and has been buying more Russian oil.

A Russian nuclear strike would likely lead both countries to reassess their current relationship with Moscow.

"Russia would really find itself completely isolated on the world stage," Bunn said.