Differentiated membership may offer a pragmatic path to EU enlargement, but it risks undermining unity in a multipolar world. Stefan Telle argues that without investment in centre-formation and shared political identity, the EU could drift into transactionalism, weakening its global voice and eroding the ideal of an ever-closer union

Differentiated membership is gaining traction as a solution to the EU’s enlargement dilemma. It offers a flexible framework for integrating candidate countries, particularly those vulnerable to Russian aggression, without demanding full and immediate compliance with all EU obligations. Yet this pragmatic approach brings strategic trade-offs. If differentiation expands without a renewed commitment to political cohesion, it may accelerate internal fragmentation and weaken the EU’s global standing. 

Enlargement in the shadow of war 

Russia’s full-scale invasion of Ukraine in 2022 reignited the urgency of EU enlargement. Granting Ukraine, Moldova, and other frontline states a swift path to membership now serves not only democratic ideals but also the Union’s geostrategic interests. Yet the existing accession framework, based on meritocratic conditionality, makes rapid enlargement implausible. The EU thus faces a dilemma: either dilute standards for the sake of speed or accept reduced influence and stability in its immediate neighbourhood. 

Under a differentiated membership model, countries that commit to EU values could join quickly, while participation in specific policies would depend on their capacity to comply with EU rules

In his contribution to this blog series, Frank Schimmelfennig proposed differentiated membership as a way out of this enlargement dilemma. Under this model, candidate countries that credibly commit to EU values could join quickly, while participation in specific policies would depend on their capacity to comply with EU rules. Differentiated membership, in this reading, is not revolutionary but evolutionary.

In the past, differentiation has enabled landmark achievements like the Schengen Area and the Eurozone, and helped hasten the Eastern enlargement of the EU. By expanding this logic, the EU could address urgent geopolitical needs without abandoning its ambitious policy standards. 

Differentiation and the risks of fragmentation 

Yet, as I argue here, the costs of normalising differentiation at the level of institutional rights and obligations are higher than often assumed. One reason is that differentiation significantly increases the diversity that the EU's political system needs to handle. Another is that differentiation is not simply a tool for those who want to do more, but also for those who want to do less.

By design, differentiation introduces institutional asymmetries, between fast and slow movers, willing and unwilling states, capable and less capable members. Temporary exclusions can harden into structural divides. Even when new member states meet formal obligations, domestic politics in older member states can stall their full inclusion, as we saw with Romania and Bulgaria’s prolonged exclusion from Schengen. Indeed, their exclusion would likely have continued without the geopolitical urgency introduced by Russia's invasion of Ukraine.

A drawback of differentiation is that exclusions and exemptions sometimes harden into structural divides

At its worst, differentiation recasts the EU as a menu of selective benefits, rather than a political community founded on solidarity and shared purpose. Brexit illustrates this danger. A long-standing patchwork of British opt-outs enabled a transactional view of EU membership, one that saw integration as a cost-benefit calculation rather than a commitment to a common path. The UK’s final push to retain economic privileges, such as a comprehensive access to the Single Market, while rejecting political obligations, was consistent with the logic of differentiation, but incompatible with the ethos of ever-closer union. For this reason, the EU rejected the UK's requests as cherry picking. 

The trade-off: flexibility versus political cohesion  

Expanding the practice of differentiation to institutional rights (i.e., no veto rights, no Commissioner), as Frank Schimmelfennig proposes, would further weaken the EU’s political community and strengthen the transactional logic of integration. It would turn the EU into a strongly asymmetrical (con-)federal arrangement. Asymmetric federations typically emerge because of a lack of common identification among the units and with the centre. Making institutional rights negotiable might lead to a situation in which the differences between full and differentiated membership are more pronounced than the differences between EU member states and non-member states participating in EU policies:

Moreover, the temptation for some existing member states to 'opt down', reducing their obligations in exchange for limited institutional privileges, could grow. Already, the hard-right Dutch government has expressed interest in withdrawing from aspects of EU migration policy. Economic stagnation and the rise of nationalist parties across Europe may amplify calls for looser, à la carte integration. Can a shared commitment to values alone hold such a complex and increasingly asymmetric union together?

Reinforcing the centre 

If differentiation is to become a pillar of EU enlargement, it must be accompanied by a serious investment in centre-formation. The EU needs not only differentiated pathways to entry but also a reinforced core that can sustain the weight of widening. 

This means building stronger democratic institutions at the centre to legitimise redistributive mechanisms and uphold common values. It also requires bold financial tools, such as joint debt issuance, to support cohesion across a more diverse union. Treaty reform may be unavoidable. 

Building stronger democratic institutions at the centre of the EU will legitimise redistributive mechanisms and uphold common values

The strategic rationale is clear. In a multipolar world shaped by Russian aggression and American ambivalence, the EU must speak with one voice. It cannot do so effectively if its internal architecture is built on opt-outs, exceptions, and asymmetry. As Jacques Delors warned in 1985, without institutional consolidation, the EU risks becoming an 'unidentified political object', an entity too loose to project influence, too fragmented to deliver solidarity. 

The case for dual strategy 

Differentiated membership may be necessary to meet the urgency of enlargement. But it is not sufficient to secure the future of European integration. The EU must pursue a dual strategy: flexibility to effectively respond to the current security crisis, combined with strong measures to reinforce cohesion among all member states of the European Union. 

This moment demands more than technocratic adaptation; it requires political vision. Enlargement and deepening are not opposing paths. If coordinated wisely, they can be mutually reinforcing. The alternative is a Union defined less by its unity than by its capacity to manage division. And in a world of hard power and fast-moving crises, that may not be enough.

No.13 in a Loop series on 🧭 EU enlargement dilemmas