President Vladimir Putin has drawn a "red line" for the United States and its allies by signalling that Moscow will consider responding with nuclear weapons if they allow Ukraine to strike deep inside Russia with long-range Western missiles.
But some in the West are asking: does he actually mean it?
The question is critical to the course of the war. If Putin is bluffing, as Ukraine and some of its supporters believe, then the West may feel ready to deepen its military support for Kyiv regardless of Moscow's threats.
If he is serious, there is a risk - repeatedly stated by Moscow and acknowledged by Washington - that the conflict could turn into World War Three.
In the latest in a long series of warning signals, Putin on Wednesday extended the list of scenarios that could lead to Russia using nuclear weapons.
It could do this, he said, in response to a major cross-border conventional attack involving aircraft, missiles or drones. A rival nuclear power that supported a country attacking Russia would be considered a party to that attack.
Both those criteria apply directly to the situation that would arise if the West allows Ukraine to strike deep inside Russian territory with Western long-range missiles such as U.S. ATACMS and British Storm Shadows, something Putin has said would need Western satellite and targeting support.
"It was a very clear message: 'Don't make a mistake - all these kind of things may mean nuclear war,'" said Nikolai Sokov, a former Soviet and Russian diplomat.
Bahram Ghiassee, a London-based nuclear analyst at the Henry Jackson Society think-tank, linked the timing of Putin's remarks to Ukraine's lobbying of the West for long-range missiles and the fact that President Volodymyr Zelenskiy is making his case to U.S. President Joe Biden this week.
"Putin is saying: just stop it right there," Ghiassee said.
'NUCLEAR BLACKMAIL'
Reaction from Kyiv was swift, with Zelenskiy's chief of staff accusing Putin of "nuclear blackmail".
"In my opinion, this is yet another bluff and demonstration of Putin's weakness. He will not dare to use nuclear weapons because that will make him a complete outcast," Anton Gerashchenko, a former adviser to Ukraine's internal affairs minister, said on X.
U.S. Secretary of State Antony Blinken said Putin's warning was irresponsible and poorly timed, and that it was not the first time he had been "rattling the nuclear sabre".
Andreas Umland, an analyst at the Swedish Institute of International Affairs, accused Putin of playing mind games.
"This is a psychological PR operation, by the Kremlin, without much substance. It is designed to scare leaders & voters of countries supporting Ukraine," he wrote.
Fabian Hoffmann, a doctoral research fellow and defence expert in Oslo, said he did not believe Putin's comments could be ignored, but that it was important not to overreact.
"Russian nuclear use is not imminent," he said on X. "Concern is warranted only when Russia signals actual preparations."
Hoffmann said next steps could be removing warheads from storage and pairing them with delivery vehicles for a tactical strike, before ratcheting up preparations for large-scale nuclear use by readying silos and putting bombers on alert - all of which U.S. intelligence agencies would detect.
And Russia security expert Mark Galeotti wrote: "Talk is easy and has political impact, but evidence of actual willingness to use nuclear weapons is both absent and something we can detect if it ever happens."
LOWER THRESHOLD
Nevertheless, Putin was more specific than in the past about the circumstances that could prompt nuclear use. His spokesman said on Thursday that his comments were meant as a signal to Western countries that there would be serious consequences if they participated in attacks on Russia.
At the same time, the announced changes fell short of what some hawkish commentators have been calling for. The best-known of them, Sergei Karaganov, has argued for a limited nuclear strike in Europe that would "sober up" Russia's enemies and make them take its nuclear deterrent seriously.
In practical terms, the changes extend Russia's nuclear umbrella to cover neighbouring Belarus, a close ally. They lower the threshold for nuclear use by stating, for example, that it could happen in response to a conventional strike that posed a "critical threat to our sovereignty".
Previously, the nuclear doctrine talked about a threat to "the very existence of the state".
Putin made the announcement in a four-minute video in which he was seen addressing the nine members of a security council that meets twice-yearly to discuss nuclear deterrence.
He said nuclear use was an extreme measure and Russia had always approached the issue responsibly.
Ministers and intelligence chiefs listened intently, occasionally fidgeting or shuffling papers. One participant - Alexei Likhachev, head of state nuclear corporation Rosatom - took detailed notes.
But the real addressees of Putin's message were in Kyiv, Washington and London.
Yevgeny Minchenko, a Russian political consultant, said the thrust of the revised doctrine was a blunt message to Ukraine and the West not to escalate the war further into Russia.
"If you try to kill us with your proxy's hands, we will kill both your proxy and you," was the message, he said.
Sergei Markov, a former Kremlin adviser, said the changes opened the door to Russia using tactical nuclear battlefield weapons in certain scenarios, namely against Ukraine.
"The threshold for the use of nuclear weapons has been lowered. Now it will be easier for Russia to use nuclear weapons," Markov said on his official blog.
"The reason for changing the nuclear doctrine was the threat of a full escalation by the West. The West is sure that Russia will not use tactical nuclear weapons first. Russia is now saying it is ready to do so."
Markov suggested Russia could use tactical nuclear weapons against Ukraine, or air bases in Romania or Poland if Ukrainian warplanes flew sorties from there and if Kyiv - backed by U.S. or British satellite support - used the jets to strike Moscow itself or parts of central Russia.
'NO RESPECT'
Igor Korotchenko, a military analyst who often appears on state TV, said the changes were necessary because the West had ignored a slew of previous warning signals against further escalation, including Russian exercises in the summer rehearsing the use of tactical nuclear arms.
"We see that Western adversaries no longer respect any 'red lines', believing that any acts to arm Ukraine and Western-assisted strikes against facilities deep inside Russian territory will not be met with nuclear escalation," Korotchenko told the daily Izvestia newspaper.
Vladimir Avatkov, who sits on an official body that offers advice to Putin on international relations, said announcing the changes to the doctrine had allowed Moscow to get ahead of any Western decision on missiles for Ukraine.
"Let them think now," he said on Telegram. "This is an attempt to not just warn them, but to give them back the fear that they have completely lost. And perhaps even some strategic thinking."
The changes were strongly welcomed by Russian nationalists and war bloggers, some of whom have long advocated Moscow use nuclear weapons to force a Ukrainian capitulation, and led to a discussion about what could trigger a nuclear response.
Dmitry Medvedev, deputy chairman of Russia's Security Council and a former president, warned that Putin's words should give Ukraine and the West pause for thought.
"The very change in the regulatory conditions for our country's use of nuclear components may cool the fervour of those opponents who have not yet lost their sense of self-preservation," he said in a statement.
'LIKE SMALL KIDS'
Sokov, the former Russian diplomat, said there was a palpable sense of frustration in Moscow that the West appeared deaf to its many nuclear warnings.
He said that when Russia staged three rounds of exercises this year to simulate preparations for the launch of tactical nuclear missiles, there were complaints in the media and among experts that Western countries were not paying attention.
"So now they decided to strengthen the signal," Sokov said. "Putin decided the West is like small kids, and you have to explain every small thing because they just don't get it."
Sokov said he was concerned about "loose talk" among politicians and commentators who argue that the West has crossed a series of Russian red lines with impunity - by supplying Ukraine with tanks and F-16 fighter jets, for example - and that Moscow's warnings can therefore be ignored.
In fact, he said, the West had yet to breach two red lines that Russia had spelt out clearly: sending NATO troops to fight in Ukraine, and letting Ukraine fire Western long-range missiles into Russia.
"How can we say how (Putin) is going to react, if so far we have not actually crossed any Russian red lines?" he said in a phone interview, arguing that such an approach was based on guesswork, not data."I'm really concerned about all the loose talk, precisely because we run head-on into a situation which is completely unfamiliar to us ... If you do not factor in the risks, you are likely to have a very unpleasant surprise."
