LAST November, in their biggest live-fire exercise since 2006, NATO forces repelled an imaginary attack on Estonia by a fictitious country called Bothnia. Steadfast Jazz 2013 was partly a response to huge and deliberately intimidating Russian exercises since 2009 that had caused jitters in Poland and the Baltic states. (One ended with a simulated nuclear strike on Warsaw.) It was also intended to mark a return to the 65-year-old alliance’s original priority of collective territorial defence as its combat mission in Afghanistan winds down. At the time, despite surging Russian defence spending and belligerent pronouncements from the Kremlin—including threats to attack a modest European missile-defence system under construction—NATO was searching for relevance because most Europeans had never felt safer.

Four months on, thanks to Vladimir Putin, NATO no longer has to justify its existence. The new revanchist Russia is not the existential threat that the Soviet Union was during the cold war. But as Mr Putin made chillingly clear on March 18th in his announcement of the annexation of Crimea, it is willing to use military force in support of coercive diplomacy when it feels its interests are jeopardised. The events in Crimea are tragic for Ukraine, and it is deeply disturbing for central and eastern European countries with significant numbers of Russian-speakers that Mr Putin claims the right to intervene on their behalf whenever he chooses. But the crisis has breathed life into the Atlantic partnership.

On March 26th, after an emergency meeting of the G7 to agree on new sanctions against Russia, Barack Obama arrived in Brussels, where NATO has its headquarters, bent on rededicating the Article 5 vow by alliance members to regard an attack on one as an attack on all. Nobody is yet talking about a return to the cold-war days when NATO was Europe’s bulwark against a foe with the armed might to back up an expansionist ideology. But Mr Putin has stripped NATO’s cosily secure members in western Europe of their illusions and the alliance’s next summit, in Cardiff this September, will be charged with new purpose and urgency.

NATO’s attempts since the 1990s to enlist Russia as a security partner through bodies such as the NATO-Russia Council now lie in tatters. “We see Russia speaking and behaving more as an adversary than as a partner,” says Anders Fogh Rasmussen, the alliance’s outgoing secretary-general. “Transdniestria, South Ossetia, Abkhazia, and now Crimea. What connects those crises is one big country unilaterally deciding to rewrite international rules.” The hope, especially strong in Germany, that the zero-sum geopolitics of hard power no longer had any place in Europe has been dashed. Mr Obama’s attempt to “reset” America’s relations with Russia after the 2008 war in Georgia is also over. So too, surely, is the belief he expressed in early 2012 that there was a “strategic opportunity to rebalance [ie, slash] the US military investment in Europe”. Out of the 6,000 forces committed to the Steadfast Jazz exercise, only 300 were American.

Almost nobody connected with NATO believes that the crisis created by Russia’s appropriation of Crimea will quickly fade. “We have learned to read Putin’s speeches,” says François Heisbourg, the chairman of the London-based International Institute for Strategic Studies. “He says what he does and he does what he says. His proposed Eurasian Union of post-Soviet states is a new empire. East Ukraine cannot be allowed to be part of the West because there is no Eurasian space without it. We have entered a long-lasting, deeply antagonistic relationship with Russia.”

The challenge for NATO is now to get all 28 members to agree on the nature of the threat posed by Russia to Europe’s security, and to decide how to respond. It must first live up to its commitment under Article 4 to reassure members who feel immediately threatened. Already, America has sent 12 F-16 fighters to Poland and ten F-15s to the Baltic states for air-patrols. They will be joined by four British Typhoons in April. NATO has also dispatched Boeing E-3As to monitor eastern European airspace.

Further ahead, it seems almost certain that NATO will resile from the declaration in the 1997 NATO-Russia Founding Act that it has “no intention, no plan and no reason” to place significant military assets in countries that joined the alliance after the Soviet Union collapsed. As Kori Schake, a Bush-administration defence official now at the Hoover Institution, a think-tank, says: “The ‘three no’s’ were a contingent restraint. Those considerations no longer apply.” But few would advocate moving NATO’s shrunken arsenal of short-range nuclear weapons nearer to Russia. They were originally intended to compensate for its weaker conventional forces if the Warsaw Pact launched a blitzkrieg attack on Europe, and are now seen as a dangerous anachronism. These days it is a relatively weaker Russia that regards battlefield nukes as a necessary force multiplier.

On March 23rd NATO’s Supreme Allied Commander Europe (SACEUR), US Air Force General Philip Breedlove, said the alliance would have to rethink how to respond to aggressive Russian troop movements carried out under cover of legitimate military exercises. NATO could reposition military forces and carry out exercises that would reassure allies, he suggested. A bigger question is whether America will emphasise its strategic commitment to the newly insecure Europe by permanently basing some of its ground forces in a frontline state, perhaps Poland.

Reboots on the ground

NATO will struggle to calibrate deterrence in conditions very different from those of the cold war, and to decide how much military aid to offer countries, such as Ukraine, Georgia and Moldova, that Russia is determined to prevent becoming alliance members. It could help Ukraine modernise its forces by providing training and selling it weapons on easy terms, says the previous SACEUR, now-retired Admiral James Stavridis, and offer intelligence and logistical support. “Would that be inflammatory?” he asks. “Compared to what?”

As for deterrence, it is clear what Mr Putin wants. He is out to restore Russia’s great-power status by forging a Eurasian union which is dependent on Russia for its economy and security—by coercion if necessary. He is pumping up Russia’s military budget by 40% over the three years from 2013, according to SIPRI, a research institute. The aim is not broad offensive operations against NATO, but to be able to make swift, targeted interventions in Russia’s “near-abroad”, backed up by the threat of nuclear escalation.

After years of squeezed defence budgets and counter-insurgency campaigns in Afghanistan and Iraq, the West’s technological superiority is no longer as overwhelming as it once was. But its capabilities still far outstrip Russia’s. For Ms Schake, the answer to deterring Russia is simple: “We must be inflexible on Article 5, but it is no good half-caring about countries that are part-way Western.”

Mr Putin wrongly believes that NATO has never ceased to be an American-dominated alliance aimed at keeping Russia down. In fact, it has proved highly adaptable and has tried hard to enlist Russia as a partner. NATO may not yet know how to handle the threat posed by Mr Putin’s Russia. But it has overcome greater challenges before.