LONDON . For close to a quarter-century there was a basic assumption in the West about Russia: It would, with zigzags and pauses, after huffs and hesitations, gradually integrate with the Western world. Whatever the misgivings in Moscow about the expansion of NATO after the collapse of the Soviet Union, this appeared to be the course set in the Kremlin, more energetically by Dmitri Medvedev, but even by earlier incarnations of Vladimir Putin.

From Berlin to Washington, the idea was that interdependence would grow. Russian membership (now suspended) in the Group of Eight leading industrialized countries was an important sign of the direction set. Modernity would do its work, breeding openness and connectedness. Autocracy and crony capitalism would yield over time (maybe even a long time, but still) to more representative government in Moscow and law-based markets.

This view of Russia, it is now clear, was wrong. Putin has decided on another course. He has opted for confrontation with the West as the basis for Russian development and the consolidation of his own power. Perhaps it was the street protests in Moscow of late 2011. Perhaps it was a perception of Western perfidy in Libya earlier that year. Perhaps it was some inkling about a moment of American weakness. Perhaps it really was the upheaval in Ukraine. Perhaps it was simply his inner K.G.B. officer rising to the surface as the years advanced.

In the end the reasons are secondary to the reality, which is that Putin has changed direction, igniting a wave of Russian nationalism. This is perhaps the greatest strategic volte-face of the 21st century, with huge and as yet scarcely digested implications. It is Putin who has pivoted to Asia, far more than President Obama, as Russia’s $400 billion gas deal with China this year suggests. He has lost interest in the West as the magnet of Russian development, portraying it rather as the flawed and predatory civilization against which a new Russia can define itself.

Did the West lose Russia? Certainly more could have been done to reassure Moscow and knit the fabric of interdependence closer. The Cold War’s peaceful end was, after all, the work of Mikhail Gorbachev, as much as anyone’s; the ending did not have to yield a stark narrative of winner and loser, much less Western triumphalism.

But the attempt at outreach to Russia from Washington and European capitals was real. Putin’s talk of Russian encirclement is baloney. The expansion of NATO was, on balance, a wise call, laying the basis for freedom and rapid development from Poland to the Baltics, as well as paying a Western debt to abandoned and imprisoned Central Europeans that dated from Yalta. Nothing is more certain than that, absent NATO membership, events in Lithuania, Estonia and Latvia would have borne a striking resemblance to those in Georgia and Ukraine in recent years. Putin’s Russia would have bitten back in any Baltic no man’s land. Poland would not be the little miracle of development it is today without its anchor in the Western alliance.

The past, in any event, is gone. What matters now is determining how to deal with the new Russia. Karl Kaiser, an adjunct professor at Harvard’s Kennedy School, told me that in recent meetings with Russian foreign policy experts one of the most striking things was their conviction that Western economic sanctions against Moscow for the annexation of Crimea could be a useful stimulus to a more autarchic model of development that would work well for Russia. “It’s actually quite scary,” he said.

In Washington, the mood is one of exasperation. Resets with Russia: been there, done that. In Europe, where Germany played a pivotal role in the imposition of sanctions, the mood is rather different. European trade with Russia dwarfs that of the United States. Russia is a critical supplier of energy. At a deeper level, most Europeans feel long-term security on the Continent can only be assured with Russia, rather than against it. If “limited conflict” with the West, as one senior German diplomat put it to me, is now Putin’s preferred approach, how is that to be managed? Germany is in the midst of an in-depth foreign policy review to be concluded early next year. Answering that question must be one of its priorities.

The Russian annexation of Crimea was an outrage, and Putin’s stirring-up of a bloody little war in eastern Ukraine an act of boundless cynicism. The danger now is that his anti-Western turn could spill over into arms control issues. The last thing the world needs is a new Russian-American arms race.

This is not a second Cold War. It is the end of a Western illusion. Cooperation is still possible; there are signs of Russian helpfulness on Iran. But Europe and the United States have not yet framed a policy that at once resists, restrains and retains Russia. It must, for the new reality is combustible.