Not since 1945 has the House of Commons been addressed by a war leader under the bombs of a foreign power. Last night a reverent hush fell on the whole House as Volodymyr Zelensky, his eyes heavy with strain and fatigue, his mouth drawn in resolve, spoke to them by videolink.

He painted a grim picture of the plight of his people, murdered and maimed in their hundreds by Russian shells. And 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, or even to take more direct military action against Russia.

Yet so far Western governments have held out. Their reasoning is very clear: at any given moment, 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 directly at them.

They do not want to commit NATO troops to Ukraine because of the risk that they will clash with the Russians. Even a small skirmish between NATO forces and the Russian military could escalate into a nuclear exchange.

Some believe this is pure cowardice. They argue it is unthinkable that either side would ever launch nuclear weapons against each other.

Nuclear weapons have only ever been used twice in history, 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.

And nuclear weapons have become far more powerful than they were in 1945. 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.

And nuclear warheads can now be mounted on hypersonicTravelling at speeds of more than five times the speed of sound (Mach 5). missiles that travel at 27 times the speed of sound. A modern nuclear war could be over before anyone even knew it had started.

Putin might be a brutal dictator, they argue, but he is not insane. He wants to have an empire left to rule at the end of all this. So he would never actually use his nuclear weapons, even if NATO did go to war with him.

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

The process is very simple. Putin notices that NATO soldiers are getting bolder. He 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.

But Putin then thinks: if NATO is willing to risk going to war, then 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 even begins. The only way to prevent this is to launch his own preemptive strike. Now nuclear war has become probable.

And at the same time, NATO knows that Putin thinks all this, and it knows the only way of beating his strike is to launch one of its own. Now nuclear war is all but inevitable.

They argue that we cannot gamble 300,000 years of human history to save the people of 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.