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 killed - and most of the casualties that time were civilians. Half of the continent's great historic cities were reduced to ruins even without the help of nuclear weapons. 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 right 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 by the fact that troops and planes from as far away as the United States and Canada are currently being sent to Nato countries that have borders with Russia. The numbers are militarily insignificant. Their purpose is simply to remind the Russians that the alliance will protect its own members should Moscow ever decide that it has also a right to "protect" Russian-speakers in Latvia, Lithuania and Estonia.
Moscow does not actually need to be reminded of that. It has seized Crimea, and is toying with the idea of seizing more of Ukraine, precisely because that country does not fall under the Nato umbrella. And it does not belong to Nato because Nato didn't want to take military responsibility for its defence.
That was an entirely rational decision, because the Russians clearly thought Ukraine fell within their sphere of influence.
This is the first time it has been independent from Russia for any appreciable period of time in the past three and a half centuries.
Moreover, the post-Soviet governments in Kiev had been horrendously corrupt and incompetent, the country as a result is even poorer than it was in Soviet times - and the population in the eastern part of Ukraine is terrified of getting tangled up with the West because it inhabits an industrial museum whose products are only saleable in Russia. What eastern Ukrainians really fear for is their jobs, not their right to speak Russian.
All this was clear 20 years ago, and that's when Nato decided that Ukraine's independence would have to depend on Russia's good-will, not on Nato's tanks. And for 20 years Russia more or less respected Ukraine's independence, while seeking, naturally enough, to ensure that its governments were friendly.
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 about Ukraine. It is a UN member, and even China has stopped supporting the Russian position. Remember East Timor.
Gwynne Dyer is an independent journalist whose articles are published in 45 countries.