The Future of NATO’s Eastern Flank

This series—featuring scholars from the Futures Lab, the International Security Program, and across CSIS—explores emerging challenges and opportunities that NATO is likely to confront after its 75th anniversary.

In the future, NATO will bolster its eastern flank by enhancing forward deterrence, integrating advanced air and missile defense systems, and developing long-range strike capabilities to ensure the security and sovereignty of member states.

In the future, NATO will continue to undertake preparations for collective defense against Russian aggression from the east. The deterrent force will be constituted of forward-deployed troops acting as a tripwire. Concurrently, the countries situated on the eastern flank will assume a more prominent role in safeguarding their national sovereignty. These countries already belong to a select group of allies that invest more than 3 percent of national GDP into defense. The investment will be directed toward increased quantity as well as enhanced quality in order to deny the enemy in the event that deterrence fails. In addition, the eastern flank allies will concentrate on air and missile defense in order to minimize damage to their territories and populations. Some member states will endeavor to enhance their long-range strike capabilities, contributing to the implementation of conventional punishment. As a result, the allies with the greatest potential will evolve from being security consumers to becoming security providers for their neighbors.

It is unlikely that the conflict between Russia and the West will abate in the near future. This is driven by the deep conviction among certain Russian elites that Western democratic systems pose an existential threat to Russia. This perception, when coupled with the persistence of imperialistic ideologies within the Russian political landscape, gives rise to a sustained drive for the expansion of the country’s geopolitical influence. The war in Ukraine is part of a larger negotiating process in which Russia is attempting to regain control over former Soviet states in Eastern Europe that have joined NATO since the end of the Cold War. This suggests that a future military invasion of one or more NATO countries is a distinct possibility. Given Russia’s perception of its inferiority to NATO conventional forces, it is likely that it will seek to isolate any targeted country at the outset of any potential aggression. This strategy may be designed to prompt a response from allies, potentially seeking to inspire a reconsideration of commitments due to the failure of deterrence.

In the wake of the atrocities perpetrated by Russia in Ukraine, NATO underwent a transformation in its conceptualization of deterrence and defense. The strategy of defending territory by trading space for time while awaiting reinforcement from NATO is no longer a viable option. This was replaced by forward deterrence and defense, which requires an enhanced approach to the force posture of eastern flank countries, shifting toward the denial of any opportunities for aggression.

In order to deter aggression, NATO must demonstrate its cohesion and prove to the potential aggressor that all member states of the alliance enjoy the same security privileges and that all parties can contribute to the burden sharing across all domains and spectrums of conflict. It is imperative that a permanent stationing of substantial NATO forces across the eastern flank be implemented in order to reinforce the credibility of the alliance’s assertions regarding the defense of its territory. The deployment of forces available to the Supreme Allied Commander Europe (SACEUR) serves to convey a sense of commitment and introduces an element of uncertainty into the calculations of a potential aggressor regarding the tentative response of NATO in the event of a violation of any member state’s sovereignty. Concurrently, such an arrangement may serve as a stabilizing factor during the initial competition for resources between countries in the region in the event of an invasion.

As evidenced by the conflict in Ukraine, the decision to invade may be based on flawed assessments. The attractiveness of the concept of potential isolation may be reinforced by the assumption that the transatlantic bond may be under stress in the event of a sudden deterioration of the security situation in the Indo-Pacific region. Consequently, the quantity and quality of regional armed forces will serve as firm indication of a genuine commitment to the defense of the eastern flank, even in the event of an initial assumption of isolation.

It is imperative that eastern flank countries maintain elevated defense-spending levels to ensure the acquisition and subsequent sustainability of capabilities that will prove critical during the initial stages of an invasion. The most crucial aspect will be the development of military mass. Using the war in Ukraine as evidence, it is reasonable to argue that an armored brigade should be opposed by another armored brigade. The concept of the importance of mass is once again receiving attention. Consequently, countries on the eastern flank are expanding their active military forces, particularly in the land domain. Likewise, there is an effort to expand reserve forces. Optimal solutions will depend on a range of national determinants, such as the size of the population or the existing culture of homeland defense. Regardless of which approach is ultimately adopted, regional countries will continue to work on the enlargement of their ready reserves. It is evident that some regional actors, due to their larger population sizes, possess greater potential than others to generate the requisite capacity in absolute terms.

Another line of effort will concentrate on the safeguarding of the population, including military forces, and the protection of critical infrastructure against air and missile threats. Such an effort already exists along the eastern flank, although it is quite diverse in its composition. It is notable that some countries have already made significant progress in developing their own air defense capabilities, with Poland serving as a case in point. Other states have indicated a preference for participating in regional initiatives, such as the European Sky Shield Initiative. It seems probable that the trend will evolve into the formation of a regional air and missile defense initiative, which would integrate the majority of the capabilities under a single umbrella. The success of this approach will depend on the inclusion of industrial partners that are currently providing air defense capabilities in national contexts.

Those eastern flank allies with the greatest potential will continue to develop long-range strike capabilities as a means of conventional punishment. Such competences will significantly contribute to the credibility of deterrence. The initial reaction of eastern flank states to the full-scale invasion in 2022 demonstrates their determination to oppose Russian aggression, despite initial concerns from some Western countries. This implies that in the event of a failure of deterrence, eastern flank countries with long-range capabilities, in conjunction with military mass and air defense, would provide the necessary motivation for the remainder of the allies to fulfill their commitments.

Piotr Niec a military professor of strategic security studies at the George C. Marshall European Center for Security Studies and a rear admiral in the Polish navy. Benjamin Jensen is a senior fellow for Futures Lab in the International Security Program at the Center for Strategic and International Studies in Washington, D.C., and the Petersen Chair of Emerging Technology and professor of strategic studies at the Marine Corps University School of Advanced Warfighting.

Piotr Niec

Military Professor of strategic security studies, George C. Marshall European Center for Security Studies
Benjamin Jensen
Senior Fellow, Futures Lab, International Security Program