Russia's use of force to compel Crimea's defection from Ukraine behind the fig leaf of last weekend's referendum marks a new phase in the history of Europe.
President Vladimir Putin has defied the principles of international order on which post-Cold-War Europe was supposed to be built, and forced the Europeans to think seriously about their own security for the first time in 25 years.
This seems to have come as a surprise to most Europeans, but it shouldn't have. Putin has simply taken the first step to reclaim the lands that Russia lost when the Soviet Union collapsed. He once called that ''a great geopolitical tragedy'', and he is not alone. Even Russians who cheered the end of Soviet regime nonetheless deeply resented the loss of immense territories that had been part of the old Russian Empire for centuries before the communists took power. They have never really accepted that loss, and it has always been likely that sooner or later Moscow would try to reverse the redrawing of Russia's western borders in 1991.
The urge to do this has perhaps become all the stronger because Russia has achieved so little else in the decades since the communists fell. Reclaiming old glories seems all the more important to a country that can claim so few new ones. And because it has achieved so little, post-Soviet Russia has no means to win its old lands back except with the still-formidable remnants of the Soviet era's only real legacy to Russia - its military strength.
But seizing Crimea will not help Russia build a better future. On the contrary, it will further estrange Russia from its European neighbours. That will create a vicious circle in which Moscow becomes even more determined to tighten its grip on neighbours that used to be part of Russia, which only makes it more isolated from the rest of Europe, further reducing its economic opportunities, and increasing the drift to authoritarian government.
Russia's willingness to use force in Crimea sharply narrows Ukraine's choices about its future. Even if Moscow pushes no further into Ukraine this time, Kiev now lives under a constant and highly credible threat of further Russian seizures of its territory, and it knows that no one will help it stop Russia militarily if that happens. The Ukrainians are on their own.
That means no Ukrainian leader will be able to sustain for long any policy that displeases Moscow. It will be drawn closer to Moscow and further from the rest of Europe, and its political and economic prospects will darken accordingly. The best that Ukraine can hope for now is to follow Finland's example during the Cold War, using adroit diplomacy to win the maximum space possible under the shadow of Russian power. Alas for Ukraine, its post-Soviet leaders have not so far shown anything like the skill required to play that game the way the Finns did.
For Europe, too, however, this is a sobering moment. It marks the end of the post-Cold-War vision of a united European community stretching from the Atlantic to the Urals in which armed forces would no longer play any role in the relations between states. One should not mock that vision because it has been so successfully achieved over so much of Europe. But one can criticise the Europeans for so unrealistically assuming that Russia would easily sign up to it.
Now that Moscow has made it so clear that it hasn't signed up, the Europeans will have to start thinking strategically again about how they deal with Russia. So, two stark questions remain: where to draw the line beyond which they will not allow Russia to use force to build its influence, and how to make sure that line is never crossed.
These are the most serious decisions the new united Europe has ever faced. Drawing that line means deciding to fight the Russians if they cross it, and any war with Russia would be immensely dangerous. Everyone now knows that Europe will not fight for Ukraine, nor for its neighbour Belarus. Most Europeans would probably agree they must be willing to fight for Poland. The hardest choice will be over the three Baltic republics, which were Soviet territory and are now part of NATO. Sentiment will say they must be saved, but cold strategic calculation may push the other way.
Wherever the line is drawn, Europeans must show Moscow they are determined to defend it. Angela Merkel made an impressive start speaking to the German parliament last week. There can be no doubt that Germany will have to lead Europe in enforcing the limits on Russian power. But no one should doubt Europe's capacity to do so; it has five times Russia's population and eight times its GDP.
And that is why this is not a rerun of 1914 or of the Cold War. In 1914 five great powers, divided into two evenly balanced blocs, began a war that neither side could in the end win. In the Cold War a devastated, impoverished and divided Europe had to rely on the US for protection from a powerful Soviet Union. Today a united and prosperous Europe faces a much weakened Russia. They have nothing much to fear if they respond wisely to Russia's challenge.