Vladimir Putin put Russia's nuclear forces on high alert during a fourth day of bloody fighting in Ukraine yesterday as the West stepped up its efforts to punish Moscow for starting Europe's biggest military offensive since 1945The end of World War II which was the deadliest military conflict in history. About 70 to 85 million people died as a direct result of the war. . 

Putin's move came after US and western allies agreed to impose sanctions on the Russian central bank and eject a group of the country's lenders from the SwiftA Belgian cooperative society that is almost solely responsible for managing international bank transfers. In 2021 it managed an average of 42 million messages per day between 11,000 members. messaging system - which is crucial for global payments - in some of the toughest such measures taken against a G20The G20 (or Group of Twenty) is an international forum consisting of the government and central bank governors of 19 countries plus the European Union. Only former US President Obama, who was meeting the Queen, did not attend Prince Charles's meeting. economy. 

NATO and the US condemned Moscow's decision, which the Biden administration described as a "totally unacceptable" escalation. "It is clearly potentially putting at play forces that, if there's a miscalculation, could make things much, much more dangerous," said a senior US defence official.

So what would happen in a nuclear war?

According to the Federation of American Scientists, Russia leads the world with 6,200 nuclear weapons. The US has 5,600, while France has 290 and the UK has 225. The weapons of mass destruction can kill hundreds of thousands of people at once.

Moscow's war chest includes 527 intercontinental ballistic missiles (ICBMs), submarine-launched ballistic missiles and strategic bombers.

ICBMs can get up to a top speed of four miles per second in about 10 minutes after launch, meaning the weapons could potentially reach the UK from Russia within 20 minutes.

And just how likely is it?

Mark Webber, professor of international politics at the University of Birmingham, explains that "nuclear war only becomes imaginable if Russia and NATO enter into direct conflict".

But even if there was such a conflict, that doesn't automatically mean it would go nuclear.

That's because of a principle called "mutual assured destruction". The phrase was coined in the Cold War when the Soviet Union and the US competitively built up their nuclear stocks.

The idea is that if one country launches a large-scale nuclear attack on another, it is effectively a suicide mission because both nations will end up obliterated.

That prospect is so dire that, the thinking goes, even countries that have nuclear weapons will never end up using them - so long as their enemies have roughly equivalent arsenals too.

That is the theory as it applies to "strategic" nuclear weapons - the large missiles that, if they're used, are intended to secure some long-term overarching objective like ending a war.

But what about "tactical" nuclear weapons? These are smaller devices designed to aid one side in a short-term battle situation, where the doctrine of mutually assured destruction doesn't apply.

Dr Kenton White, lecturer in strategic studies and international relations at the University of Reading, said he "would not expect Russia to use tactical nuclear weapons whilst invading Ukraine."

There is, he says, "no need, and it would immediately turn Russia into an international pariah, far worse than a conventional attack".

"I don't believe China would be willing to support their use by Russia," he added.

However, Dr White says, if "President Putin's expansion over-reaches itself with an invasion of the Baltic States - Estonia, Latvia and Lithuania - there is a possibility that he might use tactical nuclear weapons to avoid defeat".

Nevertheless, we are a long way from this hypothetical scenario.

Yes: There would be no point in having nuclear weapons if using them were not possible. The fact that the possibility is so dreadful is what makes it likely they will never be used. 

No: In so far as most of the world is run by people who are humane, rational and kind then nuclear war is actually not possible. One crazy dictator will not get far against seven billion people who want to preserve the planet. 

Or: One can imagine the use of a tactical nuclear weapon in a particular situation. This would be horrific but it would not amount to nuclear war and its effects would be limited.