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.