The matter of Russian nuclear warfare is top-of-mind after Vladimir Putin announced he has put his nuclear deterrents on "high alert".
Speaking on Russian TV on Sunday night, Putin said he had put "deterrent forces on special combat duty".
The announcement stoked tensions as the Russia-Ukraine war entered its second week.
Russia invaded Ukraine on Thursday, February 24, and since then, more than 2,000 civilians have died, Ukraine's state emergency service said, although that figure has not been independently verified.
Boris Johnson has accused the Russian president of war crimes and Downing Street says "horrific acts" are occurring on an hourly basis.
But what exactly does Putin's statement mean? The Mirror examined the possibility of Putin pressing the nuclear button and his state of mind.
Will Russia use nuclear weapons?
It remains unlikely, but Russian foreign minister Sergei Lavrov's threat that a "Third World War will be nuclear, and devastating" will not inspire confidence.
The military doctrine of Mutual Assured Destruction means a full-scale nuclear war would trigger the complete annihilation of both sides, so, as well as inflicting mass casualties and widespread devastation, it would be pointless.
However, there is always worrying scope for dangerous miscalculation by either side, or a protagonist who is reckless and makes bad decisions.
What is Putin's state of mind?
The 69-year-old Vladimir Putin has been in power for nearly 23 years - 19 of them as president and four as prime minister.
He has spent the best part of two years largely cut off from the outside world as he shielded himself from coronavirus, advised by just a small inner circle of acolytes.
Reports claim officials due to meet the Russian president are forced to self-isolate for two weeks before having their audience.
There are also unconfirmed claims Putin may take steroids, with his accusers pointing to the recent puffiness of his face.
Used over long periods, steroids can increase moods and affect sleep.
No10 last week refused to offer a "clinical assessment" of Putin’s state of mind, after Defence Secretary Ben Wallace claimed he had "gone full tonto".
Would Putin actually use nuclear weapons?
His televised address last week did little to reassure western viewers he had full control of his faculties.
Putin's senior advisers suggest no-one close to Putin can speak candidly to him - raising the frightening prospect that even if his finger hovered over the metaphorical red button, there might be no-one ready to intervene.
Western figures have been very clear they do no think he is behaving "rationally".
When was the last time nuclear tensions were this bad?
Two Cold War incidents brought the US and Soviet Union closer than ever to the brink of nuclear war.
One was the 1983 Able Archer exercise, an annual NATO wargame which, unlike in previous years, had added realism - so much in fact that Moscow became convinced the West was preparing a first strike and loaded planes with nuclear warheads ready to respond, before tensions eased.
The other, much more serious, episode was the Cuban missile crisis of 1962. Soviet leader Nikita Khrushchev struck a deal to place nuclear weapons on the Caribbean island - within easy range of many US cities.
President John F Kennedy ordered a naval blockade of Cuba - which Khrushchev threatened to smash.
The Kremlin said it would only remove the Soviet missiles from Cuba if US weapons were withdrawn from Turkey.
As the crisis deepened, an American U2 spy plane was shot down.
But JFK did not retaliate. Similarly, the USSR did not react to a U2 flight over Siberia or the US navy forcing a Soviet submarine to surface.
Thirteen days after the drama began, tensions eased. Kennedy had kept nukes out of Cuba, the Soviets had won the removal of missiles from Turkey.
Nuclear war was averted.
What are the strengths of Russia and NATO's nuclear forces?
Russia has the world’s biggest nuclear warhead stockpile with 5,977, according to to the Federation of American Scientists.
In comparison, it says the US has 5,428 while France has 290 and the UK has 225.
The Bulletin of the Atomic Scientists says the Russian arsenal includes 4,447 warheads, of which 1,588 are deployed on ballistic missiles and at heavy bomber bases.
What nuclear weapons does the UK possess?
The UK’s only remaining nuclear weapons are carried by four Vanguard-class submarines deployed on Operation Relentless, which has run since 1969.
Under the policy of continuous-at-sea-deterrence, at least one boat is always on silent patrol.
The submarines are armed with the Trident D5 II system.
Last year the UK announced it would raise the ceiling on its nuclear warhead stockpile from 180 to 260.
