"The prospect of nuclear conflict, once unthinkable, is now back within the realm of possibility."

This is the judgment of Antonio Guterres, Secretary-General of the United NationsAn international organisation focused on keeping peace. Its refugee agency, the UNHCR, aims to assist refugees around the world.. The invasion of Ukraine has brought the world closer to nuclear catastrophe than we have been since the height of the Cold WarA period of diplomatic conflict between blocs led by the USA and the USSR that lasted from 1947 until 1991, when the USSR collapsed..

Over the weekend, Russia fired a hypersonicTravelling at speeds of more than five times the speed of sound (Mach 5). ballistic missile for the first time since the start of the conflict. Known as the Kinzhal, or Dagger, the weapon can hit a target up to 1,240 miles away and fly up to 7,000mph. It is also capable of carrying nuclear warheadsThe device containing nuclear materials, fitted onto a missile, torpedo or bomb..

Observers point out that the weekend's attack proves Putin's willingness to use his nuclear capabilities. "It's a signal to the West", says Dominika Kunertova of the Center for Security Studies in Zurich. 

Russia has more nuclear weaponsA bomb or other weapon that uses nuclear energy to cause a big explosion.  than any other country in the world: an estimated 5,977 warheads, many of which have the ability to destroy cities in a single blast.

If Russia and the USA unleashed the full potential of their arsenals, hundreds of thousands could be killed within a single day. This could be followed by even more devastating after-effects: radiation, firestorms and a "nuclear winter" as ash clouds blot out the sun, causing worldwide famine and drastic environmental change.

Is Putin really ready to go down this apocalyptic route? Most experts think not. "His objective is not to bring the world to nuclear war," says Paul Hare, an academic and former diplomat.

But this does not necessarily mean he would not use nuclear weapons. According to intelligence reports, Russia is prepared to respond to perceived threatsRussia watchers say that Putin believes that the USA and European powers are seeking to remove him from power and that he interprets economic sanctions as preparations for war. by firing a single, smaller weapon into foreign territory as a warning: a so-called "tactical" nuclear strike. The more cornered Putin feels, the more likely he is to resort to this option.

Even a smaller strike that didn't target a populated area would have terrible consequences. It would cause severe damage from radiation and pollution and break the "nuclear taboo" that has held world leaders back from using such appalling weapons since 946The year when the USA dropped atom bombs on the Japanese cities of Hiroshima and Nagasaki, killing an estimated 200,000 people. This is the only use of nuclear weapons to date..

If Russia were to use a single nuclear bomb as a deterrentSomething that prevents people from doing something by making them afraid of what will happen to them if they do it., it is unclear whether the USA and its allies would respond in kind. The US Defense Department has role-played this scenario in recent times. One test ended in retaliation, the other did not. Princeton University ran a computer simulation in which Russia and the USA exchanged bombings and predicted a total death toll of 34 million. Not quite a nuclear apocalypse, but still arguably the deadliest event in history.

How likely is this? Most experts think the possibility is still remote - but not unthinkable. "The chance for nuclear weapons employment is extremely low," says Ulrich Kuhn of Hamburg University. "But it's not zero. It's real, and it might even increase."

Yes: Putin clearly has no concern for human life. Now he is isolated, humiliated and embattled. For such a man in such a situation, the nuclear button must be difficult to resist.

No: Putin is rash and aggressive, but not insane: he is not prepared to sacrifice Russia or himself. For all his posturing, he knows that there can be no victors in a nuclear war.

Or... The main threat is not the temperament of one individual but the fact that nuclear weapons exist at all. As long as these agents of apocalypse befoul our planet, we are only a misstep away from annihilation. We must disarm at all costs.

United Nations - An international organisation focused on keeping peace. Its refugee agency, the UNHCR, aims to assist refugees around the world.

Cold War - A period of diplomatic conflict between blocs led by the USA and the USSR that lasted from 1947 until 1991, when the USSR collapsed.

Hypersonic - Travelling at speeds of more than five times the speed of sound (Mach 5).

Warheads - The device containing nuclear materials, fitted onto a missile, torpedo or bomb.

Nuclear weapons - A bomb or other weapon that uses nuclear energy to cause a big explosion.

Perceived threats - Russia watchers say that Putin believes that the USA and European powers are seeking to remove him from power and that he interprets economic sanctions as preparations for war.

1946 - The year when the USA dropped atom bombs on the Japanese cities of Hiroshima and Nagasaki, killing an estimated 200,000 people. This is the only use of nuclear weapons to date.

Deterrent - Something that prevents people from doing something by making them afraid of what will happen to them if they do it.