Russia’s full-scale invasion of Ukraine has brought renewed attention to the boundaries of regional organizations such as the European Union and NATO. Public debate typically focuses on where these organizations should end and which countries should be admitted. New research from Koç University, however, suggests that another question may be even more consequential: How are these boundaries constructed?

In a study published open access in International Affairs, Professor Bahar Rumelili of Koç University’s Department of International Relations examines how the boundaries created by regional intergovernmental organizations can shape conflicts between member and non-member states.

Unlike national borders, the boundaries of regional organizations are not necessarily fixed, singular or mutually exclusive. An outside state may simultaneously be classified as a candidate, partner, neighbour or dialogue partner. These categories do more than define institutional relationships: they also shape expectations, hierarchies and interactions between states on either side of the boundary.

Rumelili describes these divisions as “macropolitical boundaries” and develops a framework for analysing them along three dimensions: malleability, stratification and modality.

Malleability refers to whether an organization’s boundary is open to change. A boundary is malleable when non-member states retain a credible prospect of joining or developing a closer institutional relationship. A fixed boundary, by contrast, offers little or no possibility of movement.

Stratification describes the degree of hierarchy between members and non-members. Requirements and conditions imposed by an organization can position member states as superior and outsiders as subordinate. Such hierarchy can give an organization leverage, but it may also generate resentment.

Modality concerns whether relations with outside states are organized bilaterally or multilaterally. Bilateral arrangements connect an organization separately with individual countries, whereas multilateral arrangements bring member and non-member states together within wider regional forums.

The study argues that no single form of boundary is universally ideal. Instead, the effects depend on how these three characteristics are combined.

A hierarchical boundary can help a regional organization influence an outside state when the boundary remains malleable and membership or closer association is a credible possibility. If the same hierarchy is combined with a fixed boundary, however, conditions may be perceived as permanent exclusion or external imposition, potentially intensifying conflict.

When enlargement is neither possible nor desired, multilateral relations may provide a more constructive alternative. Bringing member and non-member states together in shared institutions can encourage dialogue, socialization and the development of common rules, even when full membership is not available.

Rumelili illustrates the framework through two cases: the European Union’s involvement in the Cyprus conflict and the Association of Southeast Asian Nations’ role in the South China Sea disputes.

In Cyprus, the EU’s hierarchical structure initially created an opportunity to encourage conflict resolution through the prospect of membership. Yet Cyprus was admitted to the EU without a reunification settlement, while Türkiye’s own membership prospects gradually diminished. The EU’s boundary with Türkiye consequently became both hierarchical and increasingly fixed.

According to the study, this combination weakened the EU’s ability to encourage a settlement and contributed to new grievances between Türkiye and Cyprus. The predominantly bilateral structure of EU–Türkiye relations also limited opportunities for wider, non-hierarchical regional cooperation.

The study proposes that broader multilateral frameworks involving Türkiye, Cyprus and other regional states could create additional channels for dialogue and cooperation in the Eastern Mediterranean.

ASEAN presents a different model. Its membership boundary is relatively fixed, but its external relations are less hierarchical and more multilateral. Through forums involving China and other regional actors, ASEAN has been able to embed the South China Sea disputes within a shared diplomatic and normative framework.

This approach has not resolved the disputes, but the study suggests that it has helped ASEAN exercise influence despite significant power asymmetries. Multilateral engagement has enabled sustained dialogue and the development of common expectations concerning the management of regional tensions.

The research concludes that regional organizations should pay greater attention to the political effects produced by their institutional boundaries. If a boundary is hierarchical, it should remain sufficiently open and malleable to preserve meaningful incentives for non-members. If it must remain fixed, multilateral and less hierarchical forms of engagement may help prevent exclusion from aggravating existing conflicts.

In an increasingly volatile international environment, these choices may have consequences far beyond institutional design. How regional organizations relate to the states outside their boundaries can influence whether regional disputes are eased, frozen or intensified.