These are consequential times for the North Atlantic Treaty Organization and its member states. Justly touted as the most successful military alliance in history—having deterred Soviet aggression and prevailed in the Cold War—today NATO confronts a resurgent Russia on its borders and a metastasizing terrorist threat directed from abroad and incubated from within. Russian pressure on NATO’s perimeter, combined with terrorist mayhem on the streets of Paris, Ankara, Istanbul and Brussels, make for a formidable challenge to an alliance whose military capabilities and resolve had steadily weakened over the past two decades.

NATO Secretary General Jens Stoltenberg therefore arrives in Washington next week with weighty issues on the agenda, none more pressing than the need to bolster and reassure those member states, from the Baltic Sea to the Black Sea, most directly and uncomfortably in the crosshairs of Putin’s Russia. Stoltenberg will discuss with U.S. officials measures the alliance may adopt at its upcoming July summit in Warsaw to counter growing Russian threats. As Polish Foreign Minister Witold Waszczykowski boldly demanded in a recent interview: “We want to be protected right now.”

Vladimir Putin has been remarkably candid about his strategic goal: to roll back the historic losses suffered by the Soviet Union at the close of the Cold War, and to restore Russia’s status and prestige as a great power and peer competitor of the U.S. This ambition has profound consequences for the whole of Europe. Those now independent states formerly part of the Soviet Union—notably including current NATO Baltics members Latvia, Lithuania and Estonia—face a potential future of attenuated sovereignty and a perpetual risk of aggression, as evidenced by Russia’s invasion of Georgia in 2008, annexation of Crimea in 2014 and ongoing military campaign in the eastern part of Ukraine. Those states in the historic buffer zone of Central and Eastern Europe, formerly Soviet satellites in the Warsaw Pact and now full-fledged members of NATO, face an unending prospect of harassment and intimidation—what Poland’s Waszcykowski described as “an aggressive neighbor that is openly proclaiming the redrawing of the borders of Europe.” Finally, even historic NATO member Turkey, guardian of the Alliance’s strategic southeastern flank, faces a burgeoning Russian military buildup across its border in neighboring Armenia, while simultaneously being squeezed by Russia’s military intervention across its border with an imploding Syria.

It could indeed be argued that Putin’s ultimate aim is no less than to achieve the age-old Soviet goal of breaking up NATO and driving the U.S. out of Europe once and for all. He evidently believes that NATO’s expansion following the Cold War brought on board weak and vulnerable states who cannot easily be defended, and that NATO’s resolve to defend them is itself fundamentally weak. He views the drawdown in U.S. forces from Europe and the steep reduction in defense spending by most European allies as indicative of diminished capabilities. He sows disarray by driving refugees into Europe through reckless bombing of civilians in Syria. Exposing NATO as a hollow shell through a combination of aggression and intimidation, he seeks eventually to create in its wake a new security system in Europe with Russia at its core.

All of this underscores what is at stake for NATO today, which is above all the need to reinforce the credibility of its deterrent power. There can be, and there have been, honest differences of opinion about how much to support an independent country subject to Russian aggression such as Ukraine, which Russia covets as a kind of vassal state and for whom the U.S. and its European allies have no treaty obligations. Critics on both sides have argued persuasively both that muscular support to Ukraine was critical to checking Russia elsewhere and that doing so was futile, provocative or both. But when it comes to any potential Russian threat to NATO member states there can be no debate and no ambiguity whatsoever.

American security, and that of all of our treaty allies, depends utterly on the credibility of NATO’s Article 5 security guarantee—the certainty that the alliance collectively will spring to the defense of a member state threatened with, or subject to, attack. Both NATO members and any potential aggressor must believe that the alliance possesses the will, capabilities and intention to deliver on that solemn pledge of mutual defense. Indeed, the perception that this is so can be sufficient to dissuade a would-be aggressor in the first place. That is the most profound lesson from NATO’s success in the Cold War—credible deterrence can actually prevent conflict from occurring, and intimidation from working.

In concrete terms, what the alliance needs to do to shore up its flanks and bolster deterrence in the Baltics and Central and Eastern Europe is clear. This will involve manpower and materiel—training, exercises and the upgrading of NATO’s military presence and capabilities across those regions such that confidence in the alliance’s will and ability to defend its members is firmly established. Nowhere is this more important than in the Baltics states, but clearly Poland and its neighbors are looking for tangible reassurance as well.

Turkey, on the other hand, an ally with an impressive commitment to self-defense, is in a separate category. Its strategic importance to NATO and to the West is arguably greater today than during the Cold War, given its role as gateway to a fast disintegrating Middle East in addition to its historic adversarial relationship with Russia.

Russia has been aggressively expanding its footprint around Turkey’s borders—in Syria to the south, in Crimea and Ukraine to the north, and—most recently—Armenia to the east. Over the past year alone, the Russian military has deployed advanced drone aircraft, helicopter gunships and ballistic missiles into Armenia. Little known is the fact that Armenia has also welcomed an estimated 5,000 Russia military personnel and hosts two Russian military bases—including one within several miles of Turkey’s border. Given the chaos Turkey is facing in Syria to the south and the potentially destabilizing spillover from the conflict inside Turkey itself, as evidenced by the recent horrific terrorist attacks in Ankara and Istanbul, it is clear that Russia aims through the pressure of encirclement to intimidate if not destabilize Turkey and bend its foreign policy to its will.

Fortunately, Turkey is not an ally in need of upgraded NATO military commitments; cooperation with the U.S. in particular is robust, as evidenced by the agreement enabling U.S. aircraft operating out of Incirlik air base in southern Turkey to strike ISIS targets in Syria. What Turkey requires in the current circumstances is simply an affirmation of solidarity in view of the dire threats it is combating, and a validation of its continuing strategic importance to the NATO Alliance and our mutual security.

General Philip Breedlove, NATO’s Supreme Allied Commander, could not have defined the challenge more succinctly when in recent congressional testimony he called Russia “an existential threat to the United States, and to our European allies and partners.” Through threats and blandishments, and divide and conquer tactics, Putin seeks to destroy the alliance’s confidence and cohesion. NATO successfully countered this approach during the Cold War and, with determined U.S. leadership, we have the ability to counter it again.

But there are new dimensions to this old equation that make the challenge especially difficult. One relates to growing doubts over U.S. credibility in the world and the U.S. commitment to Europe in particular—anxieties that Putin readily exploits. The other new dimension is Putin himself, in particular the perception that he is more opportunistic, and perhaps reckless, than any Russian or Soviet leader since Khrushchev. Given his now demonstrated record of aggression in Georgia and Ukraine, and the sheer brutality of his military campaigns in Chechnya and Syria, it is clear that the most important step the U.S. can take to maintain security in Europe is to fortify and reassure any NATO member states potentially subject to Russian pressure. Standing with our allies is the surest way to deter aggression and secure peace. It is a lesson from the Cold War we should never forget.

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