Vladimir Putin's renewed threat of nuclear war, issued during a bitter and rambling speech, has revived fears that he could drop an atomic bomb on Ukraine or even a Nato ally in a so-called "tactical" strike.
"I want to remind you that our country also has various means of destruction,” he said Wednesday. "If the territorial integrity of our country is threatened, we will certainly use all the means at our disposal to protect Russia and our people.
"This is not a bluff.”
What a 'tactical' nuclear strike could look like
Nuclear weapons are generally classified as being either strategic or tactical, with the former deployed to win a war and the latter to win an individual battle.
According to the British security think tank Rusi, Russia's tactical arsenal is limited in range to around 300 miles - compared to a 3,000 mile strategic nuclear missile.
Tactical weapons are also lower in explosive yield, such as the 10kt [kilotons of dynamite] SSC-8.
However, even tactical nuclear weapons wield immense destructive power. The atomic bomb dropped by the United States on Hiroshima had a yield of around 15kt.
Russian use of nuclear weapons in such a way would be unprecedented, so it is difficult to predict how such an attack might unfold. But analysts closely following Russian nuclear rhetoric have outlined a handful of scenarios.
Lawrence Freedman, a war studies expert at King's College London, has said: "The potential targets for limited nuclear strikes [in Ukraine] are those already identified for conventional strikes - critical infrastructure more than cities."
He also pointed out in a blog post that Snake Island, which is uninhabited, could be nuked as a demonstration of Russia's power to sow fear in Ukraine and the West.
This carries its own risks, he said, such as the bomb failing to detonate and causing humiliation.
Earlier this month, Russia also conducted nuclear submarine drills in the Arctic, another staging area for a possible demonstration.
Even a low-yield nuclear strike would cause immense destruction to a major population centre like Birmingham or London. Nuclear weapons analysts say a bomb dropped on Washington would kill up to 300,000 people, not including those harmed by nuclear radiation in the wider area.
Would Putin really do it?
There are some concerns that the Russian leader may have lost his grip on reality and could resort to such a nightmare step if he continues to be humiliated by the war in Ukraine.
Boris Johnson, the former prime minister, once referred to him as an "irrational" actor who was "possibly thinking logically" about his military goals.
In terms of logistics, Putin has the power under Russian law to launch nuclear weapons in the event of an existential threat. He is said to always have at hand a "cheget", or nuclear briefcase, that connects him to the command and control of Russia's nuclear programme.
But the cheget does not contain a nuclear "red button". Instead, it transmits the order to the Russian General Staff, or central military command.
This central command has two ways of starting a launch - they can either send codes to weapons commanders or use a back-up system that bypasses all chains of command to launch land-based nuclear weapons.
If Putin opens his cheget and gives the order, one can only speculate on whether the Russian central command would follow it. There have been rumours that the Russian leader is facing fierce internal criticism for the failures in the invasion of Ukraine so far.
Perhaps an order to launch nuclear weapons on Ukraine or a Nato ally could be a step too far for even his closest generals.
Calling Putin's bluff
Western leaders have largely dismissed Putin's words as a bluff, despite his explicit insistence to the contrary.
The fact that previous nuclear threats have not been backed up somewhat undermines this. Just a few days after the invasion, he put Russia's nuclear deterrent on high alert. He also warned Ukraine's supporters that if they intervened then they would "face consequences that you have never faced in your history".
Russian propaganda networks have also made repeated and gleeful threats of nuclear annihilation against the West since the invasion began.
In perhaps the most alarming example of this, Russian state television host Olga Skabeeva said on air that Moscow should have nuked Britain on the day of Queen Elizabeth II's funeral to cause maximum chaos.
Andrey Gurulyov, a member of the Russian Duma, approvingly responded that Britain could be turned into a "martian desert".
However, nuclear analysts pointed out a subtle shift in Putin’s address on Wednesday morning.
Andrey Baklitskiy, an expert at the UN institute for disarmament research, noted that Putin threatened nuclear war "if the territorial integrity of our country is threatened".
He added: "Those statements go beyond the Russian nuclear doctrine, which only suggests Russian first use in conventional war when the very existence of the state is threatened.
"Putin adds 'territorial integrity' and [the] very abstract protection of people, independence and freedom ... coming from the person who has sole-decision making power regarding nuclear weapons, this will have to be taken seriously."
In other words, Putin could be laying a trap - if Ukraine continues its counter-offensive on occupied territory that Moscow declares "Russian" after sham referendums, then it could grant him a pretext for a nuclear strike.
The West's response
Western leaders do not seem overly concerned about the prospect of nuclear armageddon.
But if the unthinkable happened and Russian nukes were launched at the West, then it would presumably respond in kind.
Liz Truss, the UK Prime Minister, said she was "ready" to push the nuclear button during the Tory leadership contest - even if, as her interviewer put it, this led to "global annihilation".
But the response to a smaller scale nuclear strike on Ukrainian territory is less clear. It would most likely be led by Joe Biden, the US president, who refused to elaborate in detail on his response to a potential chemical or "tactical" nuclear strike in an interview last week.
"Don't, don't, don't," said Mr Biden when asked how he would react to Putin using such weapons. "It would change the face of war unlike anything since World War Two."
When pushed for details, he added: "You think I would tell you if I knew exactly what it would be? Of course I'm not gonna tell you. It will be consequential. They'll become more of a pariah in the world than they ever have been and the extent of what they do will determine what response would occur."
If radiation from a nuclear bomb in Ukraine spread to European allies such as Poland, this could trigger Article Five of the Nato defence treaty - which would see Britain, the United States and allies come to Poland's defence. How this would play out exactly also remains unclear.
Response from Russia's allies
Recent remarks from China and India, two allies who initially stayed neutral on Mr Putin's war, suggest the countries are now distancing themselves from the conflict. This suggests they are hardly eager to see yet another major escalation from Mr Putin.
China on Wednesday issued a statement urging a "ceasefire through dialogue" to resolve the invasion, suggesting that Beijing is opposed to Moscow taking any drastic steps, such as nuclear war.
"Any move towards tactical nuclear or WMD [weapons of mass destruction] - then Russia immediately loses China, their most valuable ally," said Dr Paul Dorfman, a nuclear safety expert who has advised the British government, in a Twitter post.
"In fact, Putin loses everything."