The reason was simple: they knew what would happen next, even if neither side used the thousands of nuclear weapons at its disposal. Twice in 30 years, in 1914-18 and 1939-45, a major war had been fought over almost all of Europe.
On the first occasion, they lost a generation of young men. The second time, most countries from Germany eastwards lost around 10 per cent of their populations - and most of the deaths that time were civilians. Half of the continent's great historic cities were reduced to ruins. It was a very expensive education, but the Europeans did finally learn their lesson: don't do this any more.
That is why, even as Russian tanks drive up to Ukraine's eastern borders and the Ukrainian army prepares to die in a fight it knows it would lose, nobody else in Europe is getting ready for war.
If the Russians want part or all of Ukraine, they can have it - and pay the long-term price for taking it, which would be very high. But nothing in Europe is worth blowing all of Europe up for.
Do not be alarmed that troops and planes from as far as the United States and Canada are being sent to Nato countries that have borders with Russia. The numbers are insignificant. Their purpose is simply to remind the Russians that the alliance will protect its members should Moscow ever decide that it has also a right to "protect" Russian-speakers in Latvia, Lithuania and Estonia.
Moscow does not need to be reminded of that. It has seized Crimea, and is toying with the idea of seizing more of Ukraine, because that country does not fall under the Nato umbrella. And it does not belong to Nato because Nato didn't want to take responsibility for its defence.
The collapse of the status quo is partly the European Union's fault, for demanding that Ukraine choose between closer trade and travel ties with the EU and full membership in Russia's "Eurasian Union". It is even more the fault of Moscow: President Vladimir Putin has been both emotional and opportunistic. He's scaring people, which is never a good idea.
But if he does take more or even all of Ukraine, the West will not fight him. It will just take in all the Ukrainian refugees, strengthen its eastern defences, and begin the slow process of bringing down Putin by crippling the Russian economy. That would take years, but nobody would forget Ukraine. It is a UN member, and even China has stopped supporting the Russian position. Remember East Timor.
Gwynne Dyer is an independent journalist published in 45 countries.