EUROPE NEEDS to rearm and defend itself to cope with a new military threat. The American security umbrella—in both its conventional and nuclear forms—is no longer adequate, particularly on NATO’s vulnerable eastern flanks. Indeed, the extended deterrent provided by the United States to its most exposed allies may not be well suited to inhibiting attacks similar to Russia’s recent incursion into Ukraine, which displayed all the hallmarks of the newly popular limited conventional wars—brief and decisive, violent and yet very restrained. The purpose of such conflicts is to achieve a quick fait accompli in a geographically circumscribed area through limited force—in this case, paramilitary means followed by Russian regular forces. It is difficult to deter such a threat through the promise of retaliation, which by its very nature must occur after the facts on the ground have already been changed. A threat of retaliation is simply less credible when the enemy has achieved his objective through a low-intensity action. What are needed instead are strong local military capabilities—a preclusive defense—that increase the costs of that limited attack. Europe must start to defend its border rather than indulge in the belief that the traditional formula for deterrence, based on retaliation and the extended deterrent provided by the United States, will suffice. It won’t.

WHEREAS LIMITED warfare went out of fashion in the West after Vietnam, Russia regards it as a central part of its military doctrine. It has practiced it in Georgia, Crimea and eastern Ukraine, and presumably rehearses it elsewhere. It is therefore imperative to study anew the challenges presented by such a form of sanguinary behavior. “Limited wars” have several distinctive features. First, they are characterized by self-imposed restraint in the political objective sought and the level of force used. The aggressor could escalate the confrontation, but chooses not to. The purpose of limiting the use of force is to avoid some reaction that would undermine the political objective sought in the conventional assault. In the case of today’s Russia, the purpose is to extend influence and control westward without eliciting a strong response from NATO and the United States. Moscow recognizes the clear military superiority of its main rivals and consequently desires to avoid a pitched confrontation that it would lose. Hence, its use of force is calibrated to be sufficient to conquer pieces of Ukraine but not so large and violent that it would prompt a unified political, economic and military reaction from the West.

Russia is as clear regarding what it wants to avoid as it is concerning what it wants to achieve. Moscow’s objectives are limited: a small and quick territorial grab rather than a massive invasion (at least for now). There is no drive to the capital (Kiev, in this instance) or attempt at full conquest but instead a speedy push inside the neighboring state followed by a sudden, self-imposed stop. It is a “jab and pause” style of war fighting meant to achieve a swift and limited fait accompli. A rapid conquest of Crimea or parts of eastern Ukraine is followed by a pause and apparent openness to seek a mutually acceptable negotiated settlement. But the limited objective has already been achieved, and the quick suspension of violence is a sign of the satisfaction of the original goal.

A limited war is also characterized by limited means. The aggressor state carefully tailors its methods to the goal it wants to achieve—and the reaction it wants to avoid. Minimal violence is employed. The potential for escalation is made clear but held in reserve. In the case of the Crimean invasion, the Russian operation started anonymously with unmarked troops (dubbed “little green men” by Ukrainians), an indication that Moscow was uncertain about how local Ukrainian forces would react. In the event of determined opposition, Russia maintained the option of either escalating with larger forces or, should Western powers come to Ukraine’s aid militarily, halting the operations of the unmarked troops.

The aggressor, in fact, constantly has to weigh the value of the limited objective against the risk of the rivals’ response. The higher the value of the objective, the more risk it is willing to accept. In Ukraine, it is plausible that Moscow’s desire to avoid a military clash with NATO members (including in the form of Western-armed and -trained Ukrainian forces) is greater than its desire to occupy Crimea. Russian military might is impressive when compared to that of its neighbors, but Moscow cannot sustain a prolonged conflict with Western-supported forces and certainly cannot do so against NATO member states. But the risk of a Western military response was and remains negligible, and Russia achieved its objective in Crimea with ease.

TWO MAIN challenges present themselves when crafting a response to a limited war waged by a rival. First, it is politically difficult to answer a restrained military attack. As the Crimea case illustrates, Western policy makers face significant hurdles when attempting to mobilize public opinion in support of a stiff diplomatic—much less military—response to low-scale aggression. Moreover, the tentative nature and high speed of the initial attack complicate the formation of a responding coalition, whose potential members are naturally divided as to the most appropriate answer to that limited push. The sign of a successful limited war is the absence of a strong concerted response, the reaction that the attack wanted to avoid in the first place. Russia’s self-imposed restraint in Crimea gave Moscow the advantage it sought.

The second difficulty is that when faced with a limited attack, the targeted country cannot trade space for time. The objective pursued by the attacking party is limited, most often geographically. The conquest of a small, carefully delimited piece of real estate is the goal of the aggressor, and if the defending country abandons that territory in the hope of buying time to develop a response, it ipso facto allows the enemy to achieve its objective. Consequently, defense in depth—the practice of initially yielding territory and then counterattacking—is useless in such a case. Russia does not appear interested in conducting a military conquest of Ukraine in its entirety, and seems for now to be satisfied with only Crimea and perhaps parts of eastern Ukraine, if it can hold them. In this scenario, defense in depth would simply give the aggressor what he wants, one bite at a time, as the Ukrainians quickly discovered. Whatever the reasoning behind Kiev’s initial decision not to defend its outer territories, this approach allowed Moscow to achieve its early objectives virtually cost-free—a hard lesson that prompted Kiev to switch tactics and transfer forces eastward.

These difficulties indicate that the sine qua non of a successful response to an offensive strategy based on limited war is the fielding of effective local forces capable of withstanding the initial attack. There is no alternative to local defense organized by the targeted country. Of course, it is unlikely that a country much weaker than the attacking power, as in the case of Ukraine or any other neighboring country to Russia, can defend itself alone. Local defenses only serve as a complement to—not a replacement for—extended deterrence. Without local defensive capabilities, extended deterrence is fragile, in particular in a limited offensive war; without an extended deterrent, local defense by small states facing more powerful neighbors is sacrificial.

Shifting the strategic emphasis to local defense achieves three things. First, it increases the costs of military aggression: the more difficult it is for the revisionist state to achieve the political objective sought by the limited-war format, the more force the aggressor will have to employ and the higher the risk of a stronger response by external forces. This defeats the very purpose of limited war—low-cost, low-risk revision—from the outset. The role of local defense is to force the aggressor to escalate the level of violence, which adds both military and political costs.

Second, in the event that the aggressor does attack, an effective local defense buys time for the target state, increasing the likelihood that external reinforcements will arrive before the offensive has succeeded. In a limited-war scenario, space cannot be traded for time, but time can be bought by local defensive actions. The longer it takes for the aggressor to achieve its limited territorial objectives, the greater the opportunity for external military aid to buttress the targeted country.

Third, local defensive forces permit the conflict to remain limited, an outcome that is in the interest of all parties. As William Kaufmann wrote in 1956, “To the extent, therefore, that a conflict starts with local forces clashing over local issues, to that extent will the chances of limiting it be improved.” This, paradoxically, increases the likelihood of external support for the targeted party. The security patron of a targeted small country has no interest in, and very little ability to generate domestic support for, a large-scale conflict in defense of a distant ally. If the extended deterrent is predicated on a massive military response, it is less credible in the event of a limited attack.

This is why Europeans—especially those on the eastern frontier facing a revisionist Russia—need to take their own defense seriously. The extended deterrence provided by the United States will not suffice to prevent a limited-war scenario, even in the case of a NATO member. It is plausible, in fact, to imagine a repeat of the Crimea grab in one of the Baltic states: a lighting strike with minuscule territorial objectives pursued with limited conventional means, followed by an abrupt stop to the offensive. The larger goal of such a strike, like in the Crimean case, would be to prove that the international arrangements underwriting the targeted country’s security are a house of cards. The political shadow of influence that would follow such a demonstration of power would be preferable to an outright conquest for many reasons.

The forward positioning of U.S. troops is useful for shoring up the effectiveness of American extended deterrence in the region and should be done immediately. But that step alone will not deter Russia. The deterrent aspect of this forward posture is that it puts U.S. assets and manpower in a vulnerable position—creating a so-called tripwire—thus showing commitment and creating the incentive to defend the allied country. The loss of American soldiers to an initial attack by the enemy would, so the argument goes, create powerful pressures for Washington to respond. As French general Ferdinand Foch reportedly said when asked before World War I how many British troops would be needed for the security of France, “Give me one, and I will make sure he gets killed on the first day of the war.” Or, as Thomas Schelling put it in more recent times, the purpose of placing thousands of American troops on our allies’ territory is so that “bluntly, they can die.” But what if they do not die? What if they’re never even involved because the attack is so limited—a “jab and pause” like that in Crimea—that it does not come near American forces? If the aggressor establishes a quick fait accompli, then the U.S. forces would have to be used not to defend an ally’s territory, but rather to attack an enemy that has already achieved its territorial goal and, in all likelihood, has ceased military operations. As Henry Kissinger put it, “Once the aggressor is in possession of his prize . . . the psychological burden shifts in his favor. The defender must now assume the risk of the first move. The aggressor can confine himself to outwaiting his opponent.”

THERE IS no substitute for local forces that possess the ability to protect their own borders, even if it means merely increasing the costs of aggression without hope of winning the conflict unaided. But this will require a change from NATO’s current approach to defense. As implied above, it will mean a conscious move away from the exclusive emphasis on extended deterrence that has dominated alliance strategy for decades. This approach made sense when the threat facing NATO was above the threshold of formal war, and in the immediate post–Cold War period, when the threat was negligible. But in today’s landscape, given the weak state of defenses along NATO’s eastern borders, overreliance on extended deterrence would confront NATO with the same problem now facing Ukraine, but on a wider scale. Without the ability to defend against a limited attack in its initial stages, NATO would be forced to rely on defense-in-depth techniques that would trade space for time. This is the concern that many Central and Eastern European states have—that they would have to absorb the loss of territory while awaiting relief forces that, for political or military reasons, might never come. In a best-case scenario, such an event would render an alliance in NATO’s divided political state a dead letter. In a worst-case scenario, it would turn frontline NATO members like Poland and the Baltic states into a war zone. And it also may simply let Russia achieve its limited territorial objectives, but with powerful political aftereffects. Russia does not want to march through the Fulda Gap; it simply wants to test and, if attainable at low risk, to tear down the U.S.-built and -supported European security system.

NATO needs a different defense strategy—one that retains the best features of American military protection against unlimited war but also places greater importance on ensuring the ability of frontline states to defend themselves during the critical, early phases of a Russian limited-war attack. Without abandoning extended deterrence based on retaliation, this strategy would shift the emphasis to deterrence based on preclusive defense. While similar in the sense that both seek to prevent war by changing the strategic calculation of aggression, retaliation and preclusion are different in important ways. Where the former discourages aggressive behavior by instilling fear of retaliation, the latter discourages it by removing or reducing the gain that the opponent would have achieved from aggression. Using the analogy of a schoolyard bully, deterrence is the fear of a teacher’s paddle; preclusion is equipping the weaker students with sets of brass knuckles. Preclusion works not because the opponent thinks it will lose a conflict outright—the Russians can still overcome individual frontline NATO states no matter how much they bulk up their forces—but instead because it will take more time and effort to win than the object is worth. Preclusion reinforces the effectiveness of American extended deterrence because it signals to the attacker that the target can survive long enough for the resources of its larger patron to be brought into play.

The point of the Russian “jab and pause” strategy is to make NATO’s members choose between the unsavory options of responding militarily to an already-achieved land grab (risking escalating the overall conflict) and inaction (and the resulting political self-nullification). Preclusive defense evens the odds by forcing Russia to choose between the defeat or stalling of its limited “jab,” and the adoption of a higher threshold of military violence that it is unlikely to be able to sustain. Either way, it redefines the contest in ways that allow NATO’s advantages to come into play and exposes Russian disadvantages. It prevents Russia from being able to achieve the all-important psychological advantage of the strategic-offense-cum-tactical-defense that it has used in Ukraine—the “draw[ing] of an opponent into an ‘unbalanced’ advance” that the military strategist Basil Liddell Hart identified as the most crucial determinant of success in warfare.

For preclusive defense to work, Europeans will have to get serious about defending themselves. In particular, the frontline states of Central and Eastern Europe will have to develop a capacity—and mind-set—for self-defense that they currently do not possess. One recent study by the Center for European Policy Analysis found that Russian military power outstrips the defenses of Central and Eastern European states in all dimensions by a wide margin—in land power by a factor of three to one, in airpower by four to one and in overall defense spending by ten to one. One positive side effect of the Ukraine crisis has been to increase the willingness of these states to invest in their own defense. As the recent behavior of America’s East Asian allies has shown, the return of traditional geopolitical competition has a way of awakening strategic seriousness—and reducing free riding on the United States—among vulnerable states. There are already some signs of this trend in NATO, as European defense establishments appear to be shifting emphasis to territorial defense. Poland and Estonia are already relatively big military spenders; in the period since the invasion of Ukraine, neighboring states Latvia, Lithuania, the Czech Republic and Romania have all implemented or promised significant increases, and other regional allies are considering similar options.

As the behavior of some U.S. allies during the Cold War (and in Central Europe today) has shown, it is not a foregone conclusion that all frontline states’ free riding will decrease or that local defense will become a priority on its own—even within the context of a growing threat of limited war on or near their territory. These changes are particularly unlikely if Russia maintains its low-intensity approach to the Ukraine conflict, staggers the pace of territorial acquisition in other parts of the post-Soviet space, and continues its subversive campaign inside Central and Eastern European political systems.

IF EUROPEAN states are to respond to Russia’s reintroduction of limited war by embracing the concept of local defense individually, much less adopting a preclusive-defense strategy as an alliance, they will need strong encouragement from the United States. While Washington cannot force NATO to respond to the new environment, there are things it can do to make this adaptation more likely.

To begin with, America should provide a clearer statement of its own strategy that places its requests for its allies to do more in local defense within the context of U.S. intentions and resources. At present, the widespread perception is that America is simply making it up as it goes along, trying to hold together the U.S.-led global system on an ad hoc basis with the same tools that it used in the past, except with occasional adjustments in geographic emphasis. The flat-footedness of the U.S. response to the invasion of Crimea, after years of asserting the strategic imperative of shifting attention to Asia, only deepened this impression. In such a context, and amid cuts into the muscle tissue of America’s own capabilities, requesting allies to spend more looks dangerously close to outsourcing responsibility for problems we ourselves cannot afford (and do not wish) to confront. Such an approach creates the opposite of incentives for local defense—it fuels a suspicion that “America is leaving” and that, rather than risking a hopeless defense on their own, vulnerable states would be better off avoiding actions that might antagonize the nearby aggressor (Russia). The perception of American disengagement, and thus of a weaker extended deterrent, will not stimulate exposed allies to engage in more serious efforts at local defense.

These impressions and tendencies can ultimately only be countered by having and implementing a workable strategy. NATO’s Strategic Concept has ceased to carry the credibility for playing such a role. Washington can begin to address this problem by producing an umbrella concept that outlines the seriousness of new threats like limited war, states its resolve for countering them, and explains how U.S. and allied capabilities could plausibly be employed in tandem to ensure continued stability. Allies need to understand, in unambiguous terms, that while we may be cutting back, we also have a strategy for reshaping the U.S. military at a doctrinal and technological level that sustains stability in their region. It needs to be clear to them that the success of this strategy requires local defense on their part. An implicit bargain would include U.S. investments in upper-tier capabilities like naval, air and nuclear assets paired with local investments in conventional land power sufficiently robust to create local “no-go” zones until U.S. forces arrive. Such a bargain would need to be buttressed by the physical presence of American assets and manpower—small garrisons at the frontier to show U.S. commitment and make the use of its more mobile and lethal power credible.

Most importantly, the United States needs to figure out how to create the right incentives for allies to invest in local defense. It is one thing to tell states to do more for their own defense, as recent U.S. secretaries of defense have done again and again, and another to give them real incentives to create robust indigenous militaries and avoid free riding. It’s not enough for states to be exposed to a threat, as advocates of “offshore balancing” have long argued; they must also know that they have a reasonable chance of success in pursuing the option of resistance. If NATO is going to persist in its current split into two tiers of the serious and the unserious, we might as well stack the incentives to make the former behavior profitable—and be explicit about it. Article 5 of the North Atlantic Treaty should remain the life insurance of a NATO country’s security relationship with the United States—a safety net in the event of a catastrophic, full-scale assault on a member of the alliance. But, as we observed, Article 5, and the American extended deterrent that underpins it, is less credible and effective when dealing with a quick and limited incursion. Hence, the United States should devise a “matching” strategy—a kind of geopolitical 401(k): for those allies that spend a certain amount on local defense, we will “match” their efforts in the form of commitments or agreements over and above our commitment to extended deterrence under Article 5. This could be broadened at an alliance level, if member politics allow, to create a new clause in which the alliance’s four largest economies agree to match the defense contributions of its four most geopolitically exposed members (e.g., Poland and the Baltic states) on some basis, whether through defense subsidies, technology sharing, access to sensitive weapons or troop contributions.

The “matching” approach increases the risk for those states that decide not to shore up their defenses. But, unlike a U.S. retrenchment that abandons allies to a more dangerous scenario, it also establishes clear rewards for those who decide to contribute in a meaningful way to their own security. An increased risk alone may tilt some frontier states toward the revisionist neighbor, Russia; the possibility of a reward restores the balance and gives a clear alternative to the local leaders. Further steps could include the offer of rebated surplus U.S. military equipment (artillery, tanks and fighters) to eastern NATO members, the creation of light frontier forces to give the Baltic states time to mobilize in the event of a crisis, and—over time—the creation of Swiss-style self-defense doctrines among exposed allies that would deter Russian aggression by driving up the costs of conflict at the local level.

Ultimately, the war in Ukraine demonstrates that NATO must find an effective way to deal with the revived threat of limited war. The West faces similar tactics from China in the South China Sea. Whatever form it takes, the key is to shift the focus from extended deterrence as a solution to all the alliance’s security needs to a preclusive-defense mind-set that raises the costs of limited war, mainly by incentivizing increased investments in local defense. Such an approach would prioritize the strategic resilience and survivability of NATO’s frontline states as the ultimate determinant of the alliance’s survival. It would explicitly seek to alleviate these states’ reemerging security dilemmas by both their own and other members’ contributions in the full spirit of the North Atlantic Treaty while shifting intra-NATO requirements to match a profoundly altered threat landscape. Doing so would help to support the creation of a new defense posture that, while difficult to imagine in its details now, is indispensable for ensuring the relevance and survival of NATO in a new and in many ways more dangerous era.

Jakub Grygiel is the George H. W. Bush Associate Professor of International Relations at Johns Hopkins University’s School of Advanced International Studies. A. Wess Mitchell is president of the Center for European Policy Analysis.