This week will see the 78th anniversary of the only time nuclear weapons have ever been used. On 6 and 9 August 1945, the US air force dropped two atom bombs on the Japanese cities of HiroshimaA Japanese city destroyed by an atomic bomb in World War II. and NagasakiA city in Japan that was hit by an atomic bomb during World War Two. .

It is not known how many people were killed in the blasts, but it is estimated to be anywhere between 129,000 and 226,000.¹ Around half were killed in the blast: vapourised, incinerated or crushed. The rest died slow deaths from injuries and radiation sicknessDamage caused to the body after it receives a high dose of radiation in a short amount of time. It can be life-threatening. .

Today those two bombs would be in the smallest category of nuclear arms, known as "tactical nuclear weaponsNuclear weapons that are meant to be used to make small gains on the battlefields, rather than to destroy whole cities. ". In other words, they are roughly the same size as the weapons that Russia is today threatening to use in Ukraine.

Over the weekend, Vladimir Putin's second-in-command Dmitry MedvedevA Russian politician and former president and prime minister. stated that if Ukraine succeeds in taking some Russian territory, there would "simply be no other way out" than the use of nuclear arms.

It is not the first time Russia has made such threats. But as Putin's position weakens, experts fear it is getting more likely he will push the button.

In fact, we have faced these issues before. After World War Two, the USA adopted a strategy called "massive retaliation". Even the smallest conventional attack on the US would provoke a devastating nuclear response.

This was based on the idea that the promise of massive retaliation would work to deter anyone from attacking the US.

Then in 1961, John F KennedyPresident of the US from 1961 to 1963, when he was shot dead in Dallas. He was the fourth American president to be assassinated; Abraham Lincoln was the first. proposed a new policy, called "flexible response". He thought a limited nuclear war could be possible. Instead of using its entire nuclear stockpile all at once, it could use tactical nuclear weapons to give its conventional forces an edge. They would only fire all their weapons if it looked like they were going to lose the war.

That seems to be the gamble Russia is also making. Putin calls it "escalate to de-escalate". He thinks the threat of nuclear bombs alone will force Ukraine to call off its offensive. And if they do not, he could launch a small number of tactical weapons at Ukraine without provoking a response from the US, giving the Russian army an advantage in the war.

But Kennedy's critics saw a problem with this plan in the 1960s. They argued a nuclear war could never be limited.

Imagine Putin decides to drop a bomb on Ukraine. He knows the USA does not want to respond with its own nuclear salvo against Russia, but there is still a small possibility that they will.

He therefore decides to fire weapons at the USA as well, to pre-empt their response. That way there is a small chance he can hit them before they hit him.

But the Americans also know this is the calculation Putin is making. So if they even slightly suspect that he is about to attack Ukraine, they also have to assume he will hit the USA at the same time. So they come under huge pressure to attack Russia first.

That is how a nuclear war turns from a small possibility into a near-certainty, with each side trying to second-guess the other's moves.

Yes: Some of the world's finest minds have applied themselves to these questions. There is much we can learn from their conclusions.

No: The Ukraine war is something history has never seen before: a conflict between a nuclear power and a non-nuclear power that share a border and that both claim the territory being fought over. This is a completely new situation.

Or... History can remind us that we have been in these situations before and the world has not ended. But we cannot pretend we still live in the last century. Our task is not to repeat history, but to ensure it keeps going.