President Vladimir Putin, who rules the world's biggest nuclear power, has repeatedly cautioned the West that any attack on Russia could provoke a nuclear response.
Will Putin use nuclear weapons, how many such weapons does he command and how might the United States and the U.S.-led NATO military alliance respond?
WILL PUTIN GO NUCLEAR?
Much depends on how Putin perceives the threat to the Russian state and his rule.
Putin casts the war in Ukraine as an existential battle between Russia and the West, which he says wants to destroy Russia and grab control its vast natural resources.
Putin warned the West he was not bluffing when he said he'd be ready to use nuclear weapons to defend Russia. Some analysts say Putin is bluffing but Washington is taking Putin seriously.
By claiming 18% of Ukraine as part of Russia, the room for nuclear threats increases as Putin could cast any attack on these territories as an attack on Russia itself.
Russia's nuclear doctrine allows for a nuclear strike after "aggression against the Russian Federation with conventional weapons when the very existence of the state is threatened".
Many Russians live in Ukrainian territory that Putin has proclaimed as Russian, and breaking the post-World War Two nuclear taboo would not necessarily change the tactical situation on the ground.
"He is bluffing right now," said Yuri Fyodorov, a military analyst based in Prague. "But what will happen in a week or a month from now is difficult to say - when he understands the war is lost."
Asked if Putin was moving towards a nuclear attack, CIA Director William Burns told CBS: "We have to take very seriously his kind of threats given everything that's at stake."
Burns, though, said U.S. intelligence had no "practical evidence" that Putin was moving towards using tactical nuclear weapons imminently.
WHAT NUCLEAR WEAPONS COULD BE USED?
No Russian official has called for a strategic nuclear weapons strike with the weapons that were designed to destroy cities in the United States, Russia, Europe and Asia.
Ramzan Kadyrov, head of Russia's Chechnya region, said Moscow should consider using a low-yield tactical nuclear weapon in Ukraine.
Tactical nuclear weapons are essentially nuclear weapons used on the battlefield for a "tactical" purpose and which are much less powerful than the big bombs that would be needed to destroy large cities such as Moscow, Washington or London.
Such weapons can be dropped from planes, fired on missiles from the ground, ships or submarines, or detonated by ground forces.
Although Russia has specialised nuclear forces trained to fight in such an apocalyptic battlefield, it is unclear how its army of regular troops, mercenaries, drafted reservists and local militias would cope.
WHAT WOULD THE UNITED STATES DO?
As the dominant global superpower, the United States would in effect decide the response to any Russian nuclear strike.
Russia and the United States control 90% of the world's nuclear warheads. Their arsenals were built up during the Cold War, and the Soviet Union bequeathed its nuclear assets to modern Russia.
U.S. President Joe Biden's option would include a non-military response, responding with another nuclear strike that would risk escalation, and responding with a conventional attack that could involve Washington in a direct war with Moscow.
U.S. National Security Adviser Jake Sullivan said Washington had warned Moscow of specific "catastrophic consequences" if it used nuclear arms.
Retired General and former CIA chief David Petraeus said that if Moscow used nuclear weapons, then the United States and its NATO allies would destroy Russian troops and equipment in Ukraine - and sink its entire Black Sea fleet.
Putin reminded Washington that only the United States had so far used nuclear weapons in battle - in the 1945 attacks on the Japanese cities of Hiroshima and Nagasaki.
WHO HAS THE MOST NUCLEAR WEAPONS?
Russia is the world's biggest nuclear power based on the number of nuclear warheads: it has 5,977 warheads while the United States has 5,428, according to the Federation of American Scientists.
Those figures include stockpiled and retired warheads, but both Moscow and Washington have enough firepower to destroy the world many times over.
Russia has 1,458 strategic nuclear warheads deployed - or ready to fire - and the United States has 1,389 deployed, according to the latest publicly declared data. These warheads are on intercontinental ballistic missiles, ballistic missiles on submarines and strategic bombers.
When it comes to tactical nuclear weapons, Russia has about 10 times the number the United States has. Around half of the 200 U.S. tactical nuclear weapons are deployed at bases in Europe.
The U.S. tactical nuclear weapons have adjustable yields of 0.3 to 170 kilotons (the atomic bomb dropped on Hiroshima was equivalent to about 15 kilotons of dynamite).
(Reporting by Guy Faulconbridge, Editing by Timothy Heritage)
