LONDON — Set against the backdrop of escalating challenges to European security from Russia and the Middle East, NATO’s recent announcement that Montenegro is to be the transatlantic Alliance’s 29th member might seem an inconsequential footnote. This tiny Balkan state in the southern Adriatic will add just 2,000 active military personnel to the Alliance’s strength; its navy consists of a couple of ex-Yugoslav frigates. Hardly game-changing, you might think. Yet the Kremlin blew a gasket, declaring that “the continued eastward expansion of NATO and NATO’s military infrastructure cannot but result in retaliatory actions” to ensure “parity of interests.”

* * *

NATO’s intention, and the reason for Vladimir Putin’s fury, is to reaffirm that its doors are open to any European state that shares the Alliance’s aims — and that Moscow has not, and will never have, a veto. Montenegro’s accession ends a six-year pause in NATO enlargement, after the flurry of expansion following the implosion of the Soviet Union.

In announcing the decision, NATO pointedly listed other countries in the pipeline for membership: Macedonia, Bosnia-Herzegovina and, most controversially, Georgia — the formerly Soviet state on the Black Sea where Stalin was born, breakaway parts of which were “liberated” by Russian forces in 2008.

That was Putin’s first use of the tactics later employed in Ukraine — and the first sign that Russia was bent on re-imposing its “sphere of influence.” At the time, NATO governments tried not to pay too much attention, muttering that it was all very regrettable but half-inclined to blame the victim for provoking the Russian bear.

No one wanted to admit that the West’s Russia strategy had failed.

For two decades following the end of the Cold War, under the optimistic rubric of “a Europe whole and free,” they had balanced NATO enlargement with a policy of courting Russia as a partner in building continental security. A string of agreements followed, aimed at promoting military transparency and minimizing the risk of “accidental” armed confrontation.

Defense reviews focused on global risks such as those arising from state failure and “asymmetric warfare” involving non-state actors. As military budgets were cut back, forces were remodelled to support overseas deployments. No one wanted to admit that the West’s Russia strategy had failed.

As late as 2012, Washington justified President Obama’s “pivot” to Asia on the grounds that most European countries were now “producers of security rather than consumers.” Obama, the least Atlanticist president in America’s postwar history, viewed NATO as a Cold War relic and radically shrank the U.S. military presence in Europe to a mere 30,000 troops — fewer, notes Major General Robert Scales, a former commandant of the U.S. Army War College, than the number of police in New York City. With the departing military went much of their warfighting kit.

* * *

Then, in March 2014, Russia invaded Crimea, annexing it within 30 days on the pretext of a referendum held under conditions of armed occupation. Having openly carved off one slice of Ukraine, Putin then set about fomenting and arming a separatist rebellion in the country’s Donbas region, brazenly denying the involvement of Russian troops operating without identifying insignia, while asserting Russia’s right to act wherever the interests of Russian speakers are threatened.

Russia has adopted, with significant success, a form of hybrid warfare that uses local proxy fighters backed by Russian special forces and heavy weapons, combined with a strategy of intimidation, cyberwarfare, economic pressure and a massive barrage of disinformation designed to keep foreign governments in a state of indecision.

Ukraine is not a NATO ally — and despite that, it has managed at great cost to render Putin’s Donbas campaign far messier and, coupled with Western economic sanctions, more costly than he may have bargained for.

But the new Putin doctrine directly threatens NATO’s three vulnerable member states on the Baltic: Latvia, Estonia and Lithuania. All three have Russian minorities; and all had been forcibly absorbed into the Soviet Union nearly a century ago. Russian adventurism has thus put NATO squarely back in the core business of defending its territory and — after moving not so much as a frigate into the Black Sea in response to the annexation of Crimea — under urgent pressure to demonstrate the military credibility of the collective defense guarantee laid down in Article 5 of its founding Washington Charter.

The fact remains that Putin can move far faster than NATO can respond.

The Alliance has responded with some agility. It has put the 13,000-strong NATO Response Force on a much higher state of readiness, together with a “tripwire” brigade that could be deployed at the first sign of trouble — crucially, on the order of General Philip Breedlove, NATO’s supreme allied commander, without first going through NATO’s ponderous ritual of obtaining political consensus.

General Breedlove insists that Article 5 would be triggered not only by conventional attack, but by “infiltration by anonymous foreign forces” such as the “little green men” who materialized in Ukraine. NATO air surveillance operations in the Baltic have been beefed up and materiel is being pre-positioned.

The fact remains that Putin can move far faster than NATO can respond. Russia would have the advantage of interior lines of communication. In addition, from its heavily armed naval base of Kaliningrad, wedged between Poland and Lithuania, Russian armored brigades could cut off the Baltic states from the rest of NATO within a day while its navy based there could cut off sea links to their ports.

Moscow would of course claim that any occupation was a “temporary” move prompted by the need to protect the Russian minority. NATO would then be faced with a choice: take Moscow at its word, and destroy the Alliance’s credibility and even perhaps its existence; or invoke Article 5 and risk a war in which Putin has said that he would have no compunction about using tactical nuclear weapons. Present Russian military doctrine is that a nuclear war is winnable.

* * *

A credible strategy of deterrence is thus essential. Deterrence will not be credible without a substantially increased American commitment to European defense, whether by basing more armored units there or by holding them in readiness in U.S. bases with their kit already pre-positioned. And there is unlikely to be any such commitment from the current U.S. administration even if — and it is a big if — the major Western European nations can arrive at a common understanding of the key risks facing NATO and beef up their own military expenditure accordingly.

So far, only a very few European governments have reversed the trend toward steep cuts in defense spending; and influential NATO members such as Germany oppose any substantial deployment of troops in Eastern Europe on the ground that it would be unwise to treat Russia as an “enemy.”

In France and much of southern Europe, attention has understandably shifted from the Baltics to NATO’s Mediterranean flank.

Furthermore, in France and much of southern Europe, attention has understandably shifted from the Baltics to NATO’s Mediterranean flank. NATO planners are working on a “southern strategy,” in response not only to ISIL — the murderous Islamic “caliphate” whose tentacles reach beyond its bases in Iraq and Syria to Libya, Egypt, northern Nigeria and most recently to Afghanistan — but to the destabilization of much of the Middle East, the displacement of millions of refugees, and the awkward job of dealing with Putin’s insertion of the Russian military into the already fiendishly complex Middle Eastern equation.

Trident Juncture, the NATO wargames exercise off the Spanish coast this year, was the largest since 2002 and is to be followed by regular large-scale military drills across the region. In the framework are enhanced surveillance in the Mediterranean, closer NATO co-operation with countries, and reinforced permanent deployments in acutely vulnerable countries such as Jordan.

* * *

These are all useful steps, but the stated goal, to establish “a continuum of deterrence,” is close to meaningless in the maelstrom of the Middle East. It reflects not reality on the ground, but the Obama administration’s rooted determination to be no more than peripherally involved in the struggle to restore stability to the region.

The lessons of the campaign to suppress Al Qaeda in Iraq have not so much been forgotten as ignored. The White House is unwilling to go much beyond what it optimistically calls “containment” through air power of the highly contagious menace posed by ISIL. This is a posture, not a policy; and it has already drawn the United States into tolerating, and all but collaborating with, Iran’s Al Quds Brigade in Iraq — a devil’s pact that is alienating the Sunni, whose enlistment in the fight against the scourge of ISIL is indispensable.

The governments of Eastern and Central Europe have reason for concern.

The governments of Eastern and Central Europe thus have reason for concern. They observe that the U.S. and other Western members of the Alliance are attempting to persuade Russia to turn its military firepower in Syria against ISIL rather than President Assad’s domestic opponents. To secure Putin’s co-operation on the diplomatic as well as military front, they fear, the U.S., Germany and France might be less willing to do what is necessary to head off Putin’s encroachments on NATO’s northern flank.

At the minimum, deterrence would include deployments in these countries, preferably in multinational units, to impress on Moscow that any incursions would be met not just by small Baltic armies but by NATO as a whole.

* * *

Confronted by the conventional menace of what Germany’s former foreign minister Joschka Fischer describes as “the return of power politics” on its eastern borders, and the cancerous spread of Islamist terrorism to its south, the Alliance needs both an eastern and a southern strategy. Each should be distinct, each formulated with equal clarity and urgency, and neither should be allowed to eclipse the other.

In Afghanistan, NATO has gained experience in operating in the murky spaces between war and peace. It must build on it. Adapting NATO will be expensive, more expensive than most Europeans are prepared to contemplate, and will require a wide spectrum of skills and increased operational agility.

Beyond that, there is the small matter of the incumbent of the White House. Putin has re-energized the Alliance, but the fact is that NATO proposes and America disposes. The Alliance’s existential dilemma is, as it has been since 2008, that America’s current president is fundamentally indisposed to dispose.

Rosemary Righter is an associate editor at the Times of London and a commentator on foreign affairs.