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 bombsAn atom bomb rapidly releases nuclear energy. 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

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 PutinRussia's ruler since 2000. He is a strong opponent of the West.'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.

In fact, we have faced this before. 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 on nuclear war, called "flexible response".

He thought a limited nuclear war could be possible. Instead of using its entire nuclear stockpileA country's collection of nuclear weapons. 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. But Kennedy's critics saw a problem with this plan in the 1960s. They argued a nuclear war could never be limited.

If Russia dropped a bomb on Ukraine, the USA would fear it might be next. So it would feel it had to fire its own weapons at Russia. And because Russia would know this, it would come under pressure to strike 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.