On 24 February 2022, Russian forces crossed into Ukraine in what the Kremlin described as a “special military operation”, triggering the largest armed conflict in Europe since World War II. Among the many justifications given by Moscow, one was consistent throughout the conflict: NATO’s relentless eastward expansion and its open-door policy toward former Soviet states pose a threat to Russian security. Whether one accepts this framing or rejects it, it explains the central tension that has defined European security for three decades. These tensions vary between the right of states to seek collective defense and the geopolitical concerns that such a choice provokes a neighboring great power.

NATO’s open-door policy holds that any European state capable of furthering the principles of the alliance and contributing to its security may be invited to enter the alliance, as is stated in Article 10 of the Washington Treaty of 1949.[1] What began as a procedural term in a Cold War document has evolved into one of the most consequential and contentious instruments in modern international relations. Since 1999, fourteen countries have joined NATO, and most of them are Eastern European states. These states sought the alliance’s security umbrella as a protection against the very power that once dominated them.

In 1949, NATO was founded as a collective defense alliance among 12 Western democracies. Its main purpose was to deter the Soviet expansion in the aftermath of World War II. The alliance operated within a largely fixed geographic boundary for four decades. The bipolar nature of the Cold War made the division of Europe a permanent feature of the international order in that period. The collapse of the Soviet Union in 1991 shattered that order and opened a door for questions of whether the newly independent states of Eastern Europe should be permitted to join the alliance. In 1995, NATO published a study on enlargement, and it formally set out the open-door policy as an active strategy of the alliance. It established democratic governance with civilian control over the military and peaceful resolution of territorial disputes as a condition for accession.[2]

The first wave of enlargement came in 1999, when Poland, Hungary and the Czech Republic joined. In 2004, a larger expansion followed, including seven countries, such as the Baltic states, which were formerly part of the Soviet Union.[3] That has brought NATO’s borders directly into what Russia considered its near abroad. Furthermore, the turning point was the 2008 Bucharest Summit. NATO declared in the summit that Ukraine and Georgia would become members in the alliance.[4] As a response, Russia intervened militarily in Georgia and occupied South Ossetia and Abkhazia. When the war started in Ukraine in 2022, it accelerated the enlargement of NATO that it sought to prevent.

This insight examines how NATO’s open-door policy has shaped the security dynamics in Eastern Europe. It argues that the policy has functioned as a double-edged force. On one hand, it has provided credible deterrence, and it has suppressed regional conflicts for the states in the alliance. However, the policy has intensified the security dilemma with Russia and has created a dangerous ambiguity for the states that are left on NATO’s threshold. Understanding this duality is essential for assessing the past three decades of European security and for navigating the uncertainties that lie ahead in the future.

The scope and evolution of NATO’s open-door policy

NATO’s open-door policy is established in Article 10 of the Washington Treaty, which allows any European state that can uphold the treaty’s principles and contribute to regional security to be invited to join. This article originally served as an administrative protocol reflecting the alliance’s founding enthusiasm. It carried little strategic intention at the time. Only after the Cold War ended did Article 10 gain real strategic importance,[5] therefore shifting from a largely overlooked legal prearrangement into one of the most significant in international security.

The eastward expansion was primarily driven by the United States, which saw enlargement as a means of uniting liberal democratic gains in post-communist Europe and projecting stability in a region historically prone to great power competition. France and Germany were wary about expanding eastward and were opposed to the joining of Georgia and Ukraine.[6] Their concern was that it would lead to provoking Russia and uncertainty about welcoming states that have fragile democratic institutions. Furthermore, the internal tension of the alliance showed that the open-door policy is a negotiated position shaped by competing interests and threat perceptions.

Through the 1995 enlargement study and the subsequent membership action plan (MAP), the policy established a structured pathway to membership. It tied accession explicitly to democratic governance, civilian control of the military and peaceful resolution of territorial disputes.[7] In doing so, NATO has redefined enlargement as a value-based initiative and placed the open-door policy within a wider framework of the liberal international order. This framing carried significant strategic implications. It made it politically difficult for the alliance to reject countries that met the criteria while also increasing the pressure and uncertainty for the states that remain indefinitely on the threshold.

Moreover, the credibility of the open-door policy has come under increasing strain. There is a gap between the promise of membership and the actual delivery, as shown by the situations with Georgia and Ukraine. These states were told they would join the alliance but given no timeline, thus exposing the tension at the heart of the policy. Extending a promise of membership without the protection of said members creates a geopolitical zone of strategic ambiguity. As the alliance looks ahead, the main question is no longer simply who qualifies for membership, but whether the open-door policy remains a credible and coherent instrument of collective security or has become a source of the very instability it was intended to prevent.

Impact on Eastern European security dynamics

The open-door policy has profoundly reshaped the security environment of Eastern Europe, producing outcomes that are stabilizing for members of the alliance and destabilizing for those left outside. For states that successfully joined, NATO membership delivered a deep transformation in their security. The extension of Article 5 collective defense guarantees provided a reliable deterrent,[8] especially for smaller states with limited military capacity and long histories of occupation. In 2014, after Russia’s annexation of Crimea, NATO later responded by establishing the Enhanced Forward Presence (EFP) in a couple of Eastern European states that include Estonia, Latvia, Lithuania and Poland. The EFP meant the arrangement of multinational forces on a rotational basis to raise the cost of any potential aggression.[9] These states are highly vulnerable given their geographic exposure and Russian-speaking minorities. The arrangement embodied a tangible and consequential security benefit.

Beyond deterrence, the open-door policy generated major security benefits through the process of accession itself. The conditions rooted in the Membership Action Plan requiring democratic governance, civilian control of the military, and resolution of bilateral disputes encourage wide-ranging institutional reforms across Eastern Europe. Poland, Romania, and the Baltic states undertook extensive civil-military reform as part of their accession processes. These reforms strengthened their defense sectors and democratic institutions.[10] Therefore, NATO’s open-door policy served as an external support for countries that were once under communism. The policy helped stabilize their developing democracies and placed these states within a system that emphasized shared values of democracy and accountability. This process produced security benefits beyond just individual states. It promoted a more stable and unified regional order among NATO’s Eastern European members.

On the other hand, the same policy that unified security within the alliance simultaneously created a deepening polarization between member and non-member states. The increasing eastward expansion of NATO created a sharply defined boundary between those states given collective defense guarantees and those left in an exposed, uncertain zone. Ukraine, Georgia, and Moldova are prime examples. These states have expressed Euro-Atlantic aspirations, yet have found themselves in a position where they are too strategically significant for Russia to allow them to join but insufficiently prioritized by NATO to be admitted. This structural ambiguity has produced vulnerability. Russia has taken advantage of this security gap along NATO’s eastern borders by using ongoing unresolved conflicts in South Ossetia, Abkhazia, and Transnistria to keep neighboring countries unstable and prevent them from moving closer to NATO membership.

The long-term effects of this polarization have been significant. Defense spending across Eastern Europe has risen sharply, driven by threat perceptions that already existed but have been severely intensified by the war in Ukraine. Poland shares borders with both Russia’s Kaliningrad exclave and Belarus, and has committed to raising its defense expenses to 4% of GDP, which is the highest target of any NATO member.[11] Meanwhile, the militarization of NATO’s eastern border has contributed to a regional security dynamic characterized by mutual suspicion, an arms race, and weakening diplomatic space. Thus, the open-door policy has created a paradox. It has made Eastern Europe more secure for those inside NATO while making the broader region more militarized and more prone to the kind of great power competition it was originally designed to prevent.

Case study: Ukraine and Eastern Europe

There is no better case that can show the consequences of NATO’s open-door policy than Ukraine. On one hand, Poland and the Baltic states completed their admission and have gained full protection of Article 5. On the other hand, Ukraine was promised membership but was never admitted, so it remained on the doorstep indefinitely. It is essential to understand the security crisis that followed.

The critical turning point was the 2008 Bucharest Summit, where NATO leaders declared that Ukraine and Georgia would become members of the alliance. Yet at the same time, they refused to grant either state a Membership Action Plan, which is the formal essential framework that had guided every previous accession. Russia interpreted the Bucharest commitment as a direct strategic threat, and within months, it intervened militarily in Georgia, occupying South Ossetia and Abkhazia.[12] The intervention served as a clear signal that Russia would not tolerate further NATO enlargement along its borders without a forceful response.

For Ukraine, the Bucharest Declaration produced a dangerous strategic condition where it carried the geopolitical liability of being identified as a future NATO member and without the security guarantees that actual membership would have given. This gap between promise and protection would prove deeply consequential. Its primary security guarantee was the 1994 Budapest Memorandum, under which it had surrendered its Soviet-era nuclear arsenal in exchange for assurances of territorial integrity from Russia and the United States. Russia violated that agreement without consequence. Following 2014, the Baltic states received reinforced military deployments and stronger collective defense commitments, while Ukraine merely received political support and limited military assistance. The difference between membership and the promise of membership had never been clearer.

The cost of this ambiguity in the policy was impossible to ignore when the Ukraine war started in 2022. Ukraine’s position on the threshold made it vulnerable and far from secure. That was a clear example of the consequence of promising a membership without following through. This is the core strategic failure that Ukraine exposed. Furthermore, Georgia suffered a Russian military intervention in 2008, shortly after receiving its own membership promise at Bucharest. Russia’s ongoing military presence in Transnistria has put Moldova under continuous pressure. Both countries have been made vulnerable by being associated with NATO’s open-door policy and being left on the threshold. On the other hand, in 2023 and 2024, Finland and Sweden joined the alliance. They have met the criteria to be admitted and bring strong democratic institutions, capable militaries, and no unresolved disputes with other states. Their admission was uncomplicated because the policy functions effectively when necessary political and security conditions are met. Otherwise, the consequences are catastrophic.

Escalation and regional tension in Eastern Europe

While NATO has consistently framed enlargement as a defensive and voluntary process, Russia has interpreted it as a deliberate strategy of encirclement. This fundamental difference in perception has created a security dilemma that has grown more dangerous with each wave of expansion.

Russia has always opposed NATO’s enlargement eastward since the early 1990s.[13] Over time, the country became more willing to act in response to the expansion. After the turning point in 2008 at the Bucharest Summit, the promise of future membership for Ukraine and Georgia crossed a red line for Moscow. Russia’s military intervention in Georgia later was a measured demonstration of how far it was prepared to go to prevent further NATO expansion along its borders. The limited international response to that intervention set a damaging example. It suggested that coercive action against NATO aspirant states would carry little consequences, and that would shape Russian behavior in the years that followed.

Also, the escalation has not been limited to conventional military conflict. Russia developed a number of hybrid warfare tactics that can destabilize both NATO members and countries that want to join the transatlantic alliance without triggering an Article 5 response. There have been many cyberattacks, disinformation campaigns, and political interference in Eastern European countries. These attacks are meant to take advantage of Russian-speaking minorities and break up societal cohesion. In 2007, Estonia experienced one of the first known cyberattacks on a state level.[14] These strategies show that they are intentionally trying to stay below the level of open conflict.

Frozen conflicts have functioned as a further mechanism for escalation. By keeping unresolved territorial disputes going in South Ossetia, Abkhazia, Transnistria, and the Donbas, Russia prevented Georgia, Moldova, and Ukraine from joining NATO. The alliance’s policy against letting in countries with ongoing territorial disputes means that these disputes serve as low-cost means to keep aspiring states unstable and unable to achieve the requirements for joining. As a result, the states closest to Russia are the most vulnerable and least protected in the region.

Additionally, tensions emerged within the alliance. Especially, Poland and the Baltic states have always sought more troops and stronger deterrence measures, while Western members have been more careful, putting political and economic concerns ahead of military commitments. Yet, Hungary has been the most obvious example of internal disagreement, keeping tight connections with Russia and consistently blocking aid to Ukraine.[15] These different views of the danger have made it harder for NATO to portray a united front, which Russia has actively tried to take advantage of.

The 2022 invasion of Ukraine marked the most severe escalation in European security since World War II. It showed that the combination of NATO’s open-door promise and its hesitation to fully commit to that promise had created conditions for catastrophic conflict. The militarization of Eastern Europe has increased since the war. Members of the alliance on the eastern border have increased their defense spending and requested permanent deployment of troops. The region is now more heavily armed and more defined by confrontation than it ever was since the Cold War.

It is clear that these dynamics escalated the tension in Eastern Europe, and that is not separate from NATO’s enlargement policy. The policy raised expectations that it did not meet. Furthermore, it drew boundaries it did not always defend and provoked responses it did not always foresee. Addressing this trajectory without abandoning the sovereign right of states to seek collective defense remains the central security challenge facing the Alliance today.

Conclusion

To conclude, this insight explores how NATO’s open-door policy is one of the most consequential instruments of post-Cold War international security. Moreover, it has examined the scope and evolution of the alliance and the impact it has on Eastern European security dynamics. The analysis focused on the case of Ukraine and its role in the regional tension. Across each of these explored scopes, the same pattern always occurs: the open-door policy has provided significant security benefits for countries that were admitted, while at the same time generating serious instability for those left at the doorstep.

NATO membership provided credible deterrence and embedded fragile post-communist institutions within a framework of shared norms and collective defense. Poland, Romania, Bulgaria, and the Baltic states are the clearest examples of this outcome. Their integration into the alliance transformed their security posture in ways that no bilateral arrangement could have replicated. The open-door policy, in this context, functioned as a powerful stabilizing force for Eastern Europe’s new democracies.

However, the same policy produced a dangerous zone of ambiguity along NATO’s eastern periphery. States such as Ukraine, Georgia, and Moldova were drawn into the alliance’s orbit without receiving its protection. They were promised membership without being given a credible pathway to it. This gap between commitment and delivery did not produce neutrality; it produced vulnerability and hostility. Russia exploited that vulnerability through military intervention and frozen conflicts designed to keep aspirant states weak and permanently outside the Alliance.

Ukraine represents the most devastating consequence of this failure. The promise made at Bucharest in 2008 was never matched by the political will to fulfill it. The result was a state that carried the geopolitical liability of a future NATO member without any of the security guarantees that membership provides. The 2022 war was the product of a structural condition that the open-door policy helped create and failed to resolve.

The main argument of this insight is, therefore, that NATO’s open-door policy has functioned as a double-edged mechanism. It has strengthened security inside the alliance while deepening instability at its peripheries. It has advanced democratic norms within while creating strategic ambiguities and encouraging undemocratic practices in its periphery. It has expanded the zone of collective defense while leaving an equivalent zone of unprotected exposure along Europe’s eastern flank.

Looking ahead, the alliance faces a fundamental question. Can the open-door policy be made more credible and strategically coherent? The accession of Finland and Sweden demonstrated that enlargement could work effectively when the right conditions are in place. Ukraine showed what happens when they are not. The point is not that the open-door policy should be abandoned, because closing it would encourage aggression and undermine the sovereign rights of states to seek their own security arrangements. The policy must be applied with greater clarity and strategic foresight. A promise of membership that cannot be honored is a liability. Until NATO resolves this tension, the open-door policy will remain as much a source of instability as it is a foundation of collective security.

1. NATO, Official Texts and Resources, “The North Atlantic Treaty,” April 4, 1949, https://www.nato.int/en/about-us/official-texts-and-resources/official-texts/1949/04/04/the-north-atlantic-treaty.
2. NATO, Official Texts and Resources, “Study on NATO Enlargement,” September 3, 1995. https://www.nato.int/en/about-us/official-texts-and-resources/official-texts/1995/09/03/study-on-nato-enlargement.
3. NATO, Organization, “NATO Member Countries,” March 11, 2024, https://www.nato.int/en/about-us/organization/nato-member-countries.
4. NATO, Official Texts and Resources, “Bucharest Summit Declaration,” April 3, 2008, https://www.nato.int/en/about-us/official-texts-and-resources/official-texts/2008/04/03/bucharest-summit-declaration.
5. Bengisu Inan, “The Impact of NATO Enlargement to Eastern Europe on US-Russia Relations,” Atlas Institute for International Affairs, September 4, 2025, https://atlasinstitute.org/the-impact-of-nato-enlargement-to-eastern-europe-on-us-russia-relations/.
6. David Cadier, Martin Quencez, “France’s Policy Shift on Ukraine’s NATO Membership,” War on the Rocks, August 10, 2023, https://warontherocks.com/2023/08/frances-policy-shift-on-ukraines-nato-membership/.
7. NATO, “Membership Action Plan (MAP),” March 28, 2024, https://www.nato.int/en/what-we-do/partnerships-and-cooperation/membership-action-plan-map#:~:text=Updated:%2028%20March%202024,review%20of%20agreed%20planning%20targets.
8. NATO, Official Texts and Resources, “The North Atlantic Treaty.”
9. NATO, Official Texts and Resources, “Warsaw Summit Communiqué,” July 9, 2016, https://www.nato.int/en/about-us/official-texts-and-resources/official-texts/2016/07/09/warsaw-summit-communique.
10. Jânis Karlsbergs, Ìirts Valdis Kristovskis, Theodor H. Winkler, Rasa Jonusaite, Lauri Almann, Igors Rajevs, Robertas Ðapronas, et al. “DEFENCE REFORM IN THE BALTIC STATES: 12 YEARS OF EXPERIENCE,” Conference-proceeding, 2003, https://www.dcaf.ch/sites/default/files/publications/documents/publication.pdf.
11. Izabela Surwillo and Veronika Slakaityte, Power Moves East: POLAND’S RISE AS A STRATEGIC EUROPEAN PLAYER, Danish Institute for International Studies, 2024, http://www.jstor.org/stable/resrep64401.
12. Kenji Iwata, “Russia’s Security Policy from the Bucharest NATO Summit and Russia-Georgia War through the Brussels NATO Foreign Ministers Meeting,” and Hiroshima University, 2008, https://heiwa.hiroshima-u.ac.jp/Pub/42/7IWATA.pdf.
13. “The Forgotten NATO Enlargement Dove in the Kremlin,” Martens Centre, September 16, 2024, https://www.martenscentre.eu/news/the-forgotten-nato-enlargement-dove-in-the-kremlin/.
14. Viljar Veebel, Illimar Ploom, and Vladimir Sazonov, “Russian Information Warfare in Estonia, and Estonian Countermeasures,” Lithuanian Annual Strategic Review 19, 2021, https://www.researchgate.net/publication/361396571_Russian_information_warfare_in_Estonia_and_Estonian_countermeasures
15. Giorgio Cafiero, “Hungary’s Rejection of Confrontation with Russia,” TRENDS Research & Advisory, October 26, 2025, https://trendsresearch.org/insight/hungarys-rejection-of-confrontation-with-russia/?srsltid=AfmBOormrAAo0IQTjDBiwCWRtryQgElKyx2gwqzTmZ2vHwTof_LySbZy.