Not since 1945 has the House of CommonsThe democratically elected house of the UK Parliament. It consists of 650 Members of Parliament.  been addressed by a war leader under bombs of a foreign power. Last night a hush fell on the House as Volodymyr Zelensky spoke to them by videolink.

He painted a grim picture of the plight of his people, murdered and maimed by Russian shells. He begged the UK to take firmer action against the state that is laying waste to his country.

His address has fuelled calls to create a no-fly zoneAn area over which certain aircraft are prevented from flying. Since they involve shooting down enemy aircraft, an NFZ in Ukraine would likely lead to war with Russia. over Ukraine.

So far Western governments have held out. Their reasoning is clear: Russia has as many as 2,000 nuclear warheadsExplosive devices based on harnessing atomic energy. A nuclear bomb the size of a conventional bomb has the power to destroy an entire city. pointing at them.

Some believe this is cowardice. They argue it is unthinkable that either side would launch nuclear weapons.

Nuclear weapons have only ever been used twice, both by the USA against Japan in 1945. Since Russia developed its own nuclear armaments in 1949, followed by Britain, France and China, it has become unthinkable to use them, for fear of retaliation.

Nuclear weapons have become far more powerful. The B83The most powerful nuclear bomb in the USA's arsenal, with a blast yield equivalent to 1.2 megatonnes of TNT. nuclear bomb is 80 times as powerful as the bomb dropped on HiroshimaA Japanese city destroyed by an atomic bomb in World War II., which killed 140,000 people.

Nuclear warheads can be mounted on hypersonic missiles that travel at 27 times the speed of sound. A modern nuclear war could be over before anyone knew it had started.

Putin wants to have an empire left to rule. So he would never use his nuclear weapons, even if NATO did go to war with him.

Others think the West must hold its nerve. They point out that the moment nuclear war becomes possible, it becomes probable, then inevitable.

The process is simple. Putin realises NATO might be willing to risk war with Russia, even though it could lead to a nuclear exchange. Now nuclear war has become possible.

Putin thinks: if NATO is willing to risk war, they must have plans in the event of a nuclear exchange. Perhaps they believe they can launch a preemptive strikeA surprise attack intended to prevent an expected enemy offensive. and end the war before it begins. The only way to prevent this is to launch his own preemptive strike. Now nuclear war has become probable.

NATO knows Putin thinks this, and the only way of beating his strike is to launch one of its own. Now nuclear war is inevitable.

They argue we cannot gamble 300,000 years of human history to save Ukraine.

Yes: Putin has become paranoid and irrational. He might even think he can win a nuclear exchange. Even if not, at a certain point the risks of not using nuclear weapons become too great: one side will have to fire them.

No: Everyone knows that nuclear weapons, if used, could wipe out human civilisation. When it comes down to it, no-one could possibly ever convince themselves to press the button.

Or... It does not matter whether or not nuclear war is probable. The outcomes of such a conflict would be so catastrophic that even if it is very unlikely to happen, we have to treat it as a certainty.