With the Russian military suffering setbacks in Ukraine, Russian President Vladimir Putin has threatened to use "all means" available, raising fears of a nuclear conflict for the first time since 1945.
US President Joe Biden set up a team of civilian and military specialists to assess risks and responses, warning Russia that nuclear war "cannot be won," he said.
Experts and officials explain the possible scenarios, according to Agence France-Presse.
What kind of attack might Putin launch?
James Cameron of the Oslo Nuclear Project rules out Putin's use of Russia's strategic nuclear arsenal capable of striking the United States, igniting what he calls a horrific nuclear war.
But Russia, the world's number one nuclear power with a stockpile of about 4,500 nuclear warheads according to estimates by the Stockholm International Peace Research Institute (SIPRI), possesses "tactical" nuclear weapons less powerful than the Hiroshima bomb.
The Russian president may decide to detonate one of these "small" nuclear weapons in Ukrainian airspace or in the Black Sea, according to the Russian nuclear "escalation and de-escalation" doctrine of using a light nuclear weapon first to excel in the event of a conventional conflict with the West.
It could also target a sparsely populated area of Ukraine or a Ukrainian military facility to terrorize the population into surrender, or even induce Westerners to persuade Ukraine to surrender, according to James Cameron, a columnist for the Washington Post.
Vehicles carrying strategic RS-24 nuclear missiles during a military parade in Moscow (European News Agency)
What could prompt Putin to use nuclear weapons?
Putin emphasized that he might resort to nuclear weapons if Russia's territorial integrity was threatened.
He did not say whether this included Crimea, which was annexed by Russia in 2014, or the four Ukrainian regions partially controlled by the Russian army in Donetsk and Luhansk in the Donbas region (east), Kherson and Zaporozhye (south), where referendums are being organized hastily to annex them to Russia. Russia.
In the opinion of a specialist in nuclear strategy in the former US Navy, Mark Kansian, this ambiguity means that the matter does not include the Donbass and the Crimea.
"There is no point in issuing a threat with such ambiguity if people are not sure whether they are really vulnerable or not," said Kansian, who is currently an expert at the Center for Strategic and International Studies (CSIS).
The US government has not observed any movement of nuclear weapons that would suggest preparation for such a strike.
"We haven't seen anything that would change our position," Pentagon spokesman Major General Pat Ryder said Thursday.
What will be the answer of the Westerners?
Since the start of the conflict in Ukraine, the US government has been trying to avoid any escalation with Russia.
Nor did NATO send troops to Ukraine, and the weapons that Kyiv's allies sent to Ukrainian forces were thoughtful to avoid the use of Western missiles to strike targets in Russian territory.
In this context, the US administration has sent several private messages to Russian leaders over the past months to discourage them from using nuclear weapons, according to the Washington Post.
However, according to Agence France-Presse, Washington must be firm even in the event that Moscow decides to conduct a limited strike in Ukraine, which is not a member of NATO.
Failure to respond firmly enough may divide allies, but it may, in particular, according to Matthew Croening of the Scowcroft Center for Strategy and Security, encourage other countries, such as China, to "think that nuclear weapons can help them achieve their goals without dangerous repercussions."
Kroenig, a former strategic adviser to the Pentagon and the CIA, suggested that the United States respond with a conventional strike on Russian forces or the military base from which the nuclear strike was launched, and by strengthening military support for Ukraine with long-range artillery systems that Washington has so far refrained from providing Kyiv.