Enlargement has once again become one of the main strategic problems of the European Union. For part of the 2010s, the process seemed to slow down, notably under the effect of institutional fatigue resulting from previous enlargements, the euro area crisis, migratory tensions and Brexit. The war unleashed by Russia against Ukraine has profoundly changed this situation. The potential accession of Ukraine and Moldova, the revival of the Western Balkans process and the prospect of a Union eventually exceeding thirty States have put enlargement back at the centre of the European project. In December 2023, the European Council opened accession negotiations with Ukraine and Moldova and explicitly linked further enlargement to the reforms necessary for the future functioning of the Union[1]. Its Strategic Agenda 2024-2029 confirms this articulation by providing for a merit-based enlargement process, conducted in parallel with the necessary internal reforms[2].
However, this return to enlargement does not constitute a repeat of the sequences of the 1990s and 2000s. The Union that could welcome new members in the coming years is much more integrated, more politicised and more exposed to power relations than the one that prepared the major enlargements towards Central and Eastern Europe. The Member States now share a deeply integrated internal market, a common currency for a majority of them, numerous common policies and a legal order whose effects cut through a large part of their public policies. At the same time, successive crises have revealed persistent disagreements on budgetary solidarity, migration, energy, relations with Russia, industrial policy and the desirable depth of integration. Adding new members therefore does not simply mean enlarging an existing organisation. It amounts to increasing the heterogeneity of a system already faced with the difficulty of transforming its level of integration into political capacity.
This distinction is fundamental. The traditional question of enlargement is external: which States can join the Union and under what conditions? The contemporary question is simultaneously internal: which Union will be able to function when it has more than thirty States? The issue is not only about the administrative capacity of the candidates to adopt the acquis communautaire. It also concerns the ability of the European institutions to decide, finance, arbitrate distributive conflicts, preserve the rule of law, produce strategic policies and maintain sufficient solidarity between more and more different members.
Enlargement thus becomes a major test of post-Europeanism. It forces the Union to determine whether it can continue to presuppose a gradual convergence of all its members towards a relatively comparable degree of integration or whether it must now organise more explicitly differentiated degrees of participation. In a larger Union, uniformity may become more difficult; but poorly controlled differentiation can, conversely, create first and second category members. The problem therefore consists in articulating three requirements: to enlarge the European political space, to preserve the coherence of the common base and to prevent increased diversity from leading to a paralysis of collective action.
1. Enlargement has once again become a geopolitical policy
European enlargement has never been a simple trade policy. From the very first stages of the construction of the Community, membership meant entry into a specific political and legal order. This function was strengthened after the Cold War. The enlargements of 2004 and 2007 helped to anchor in the European institutions States that had emerged from several decades of Soviet domination or communist regimes, by combining economic integration, democratic consolidation, the rule of law and belonging to the Western space.
However, the new phase of enlargement has an even more explicit strategic dimension. The opening of accession negotiations with Ukraine and Moldova comes in the direct context of the Russian war against Ukraine. The potential accession of these states therefore means more than a gradual alignment with the EU acquis: it is a choice of continental geopolitics. The Union thus affirms that the political membership of these countries in the European space must be able to become sustainable and institutionally irreversible.
This development reinforces the strategic importance of the Union but also increases its responsibilities. The integration of States exposed to military threats, hybrid pressures, energy vulnerabilities or considerable reconstruction needs requires the Union to be able to provide something other than a regulatory framework. It must be able to contribute to their security, support their economic modernisation, organise the infrastructure enabling their integration into the internal market and absorb the budgetary consequences of their accession.
Enlargement therefore becomes an instrument of power only if the Union has the capacity to support the space it is extending. The additional territorial, demographic, agricultural, industrial or strategic weight can strengthen Europe, but it can also increase its fragilities if the common mechanisms remain insufficient.
The Strategic Agenda 2024-2029 reflects this shift by directly linking enlargement to a stronger and more secure Europe, defence capacity building, resilience and the reduction of strategic dependencies[3].
Enlargement is therefore no longer just a policy of integration. It has become a European policy.
2. A Union of more than thirty States would not simply be a larger Union
The size of an organisation does not mechanically determine its governability. The history of European integration clearly shows this. The Communities have gone from six to nine, then ten, twelve, fifteen and finally twenty-seven Member States without integration ceasing to progress. The institutions have been adapted, qualified majority voting has been extended and the balances between States have been redefined.
The current difficulty lies less in the absolute number of members than in the combination of this number and the depth of common policies. The Union of the 1980s had neither the euro, nor Schengen in its current form, nor an area of comparable freedom, security and justice, nor contemporary competences in climate, digital or economic governance. A Union of more than thirty members would therefore have to take joint decisions in areas that were much more politically sensitive than those that structured the first enlargements.
Economic heterogeneity would also be important. The candidate countries have very different levels of development, economic structures and administrative capacities. Ukraine alone is a case of particular magnitude in terms of its population, territory, agricultural sector, industrial base, investment needs and the consequences of the war. The Western Balkans have different profiles, while Moldova has a very different size of economy and institutional capacities.
This diversity is not an argument against membership. It does, however, mean that enlargement changes the redistributive balances, sectoral interests and political coalitions within the Council. An enlarged Union will not simply bring together more states with the same preferences. It will bring together more conceptions of agricultural policy, cohesion, taxation, energy, defence or relations with external powers.
The outlook study published by the European Parliamentary Research Service in 2025 shows that enlargement can generate significant economic and strategic benefits. Under its moderate convergence scenario, the EU’s GDP could be around 8% higher by 2035 than in a scenario without further enlargement; a more ambitious scenario produces potentially greater gains. The report also highlights possible benefits in terms of competitiveness, sovereignty, critical resources and defence [4]readiness. These figures are forward-looking and should be understood as the results of scenarios, not as certain forecasts.
Enlargement can therefore increase Europe’s power potential. But potential is not yet a capacity: it depends on the political architecture that allows the additional resources to be used.
3. The decision is the first institutional problem
The question of governability appears first of all in the rules of decision-making. A larger Union automatically increases the number of preferences that can be represented in the Council. In areas governed by qualified majority, the system can continue to produce decisions despite divergences. In areas where unanimity is still required, each new State increases the number of governments with an individual blocking capacity.
The problem becomes particularly important for foreign policy, certain tax issues, certain budgetary decisions or institutional revisions. In a Europe of more than thirty states, systematically requiring unanimity can make it more difficult to quickly adopt a common strategic position. This difficulty explains why debates on enlargement are now almost systematically associated with discussions on the extension of qualified majority voting.
The solution seems technically obvious but politically much less simple. Moving from unanimity to majority voting is not only a gain in efficiency. It means that a state can be constrained by a decision that it considers to be contrary to an important national interest. This possibility is more easily accepted in certain areas of the internal market than when it comes to taxation, sanctions, migration policy or security.
Enlargement thus produces a structural tension: the larger the Union, the more it needs mechanisms to avoid deadlock; but the more diverse it becomes, the more the legitimacy of majority decision can be challenged when it affects the central functions of the State.
This paradox makes it necessary to distinguish between policies. Some can legitimately be decided by a majority of the whole. Others may require special protection of national interests. Between the two there is a third space: that of policies in which several States wish to act more deeply together without being able or willing to immediately involve all the others.
It is precisely in this space that differentiated integration becomes an institutional option.
4. Enlargement also transforms the budgetary question
A larger Union will also have to review the conditions of its financial solidarity. The European budget finances agricultural policy, cohesion, infrastructure, innovation, external policies and various strategic priorities. The integration of several states whose average income remains lower than that of the current Union would necessarily change the geography of the beneficiaries and contributors.
This problem would be particularly important in the case of Ukraine. In addition to the gradual application of the acquis, its accession would raise considerable questions of reconstruction, modernisation of infrastructure and integration of a large-scale agricultural sector into the common market. A mechanical extension of existing policies could profoundly alter the balance of the common agricultural policy and the cohesion funds.
The issue is not necessarily whether enlargement will « cost » or « return » the Union overall. The long-term economic effects may be positive, as the European Parliament’s scenarios suggest, but their distribution will necessarily be unequal across sectors, territories and periods. A policy can produce a collective gain while imposing significant costs on certain groups. It is precisely this distributive dimension that transforms an economic issue into a political problem.
Three responses are then possible. The first is to increase the resources of the European budget in order to preserve existing policies while financing the integration of the new members. The second is to profoundly reform the distribution of current expenditure. The third could be to develop certain differentiated financial instruments or common capacities around groups of states ready to finance more of a particular priority.
These options do not have the same institutional consequences. Significantly increasing common resources would bring the Union closer to a more federal budgetary capacity. Reducing common policies in favour of national budgets would reinforce a more confederal logic. Multiplying the capacities financed by variable groups would produce a more differentiated Europe.
Enlargement therefore acts as a revelation of the constitutional choices implicit in the budget.
5. The history of enlargement shows that differentiation can facilitate integration
The Union already has considerable experience in enlargement-related differentiation. The new members have never necessarily applied all EU rules under exactly the same conditions from the first day of their accession. Transitional periods, temporary exemptions and specific mechanisms have regularly been used to take account of differences in economic and administrative capacity.
Frank Schimmelfennig and Thomas Winzen show that enlargement is precisely one of the main drivers of European differentiation. They distinguish between an « instrumental » differentiation, mainly associated with differences in wealth, governance and the ability of candidates to adapt, and a « constitutional » differentiation, which is more linked to the political will of certain states not to participate in further integration into central functions of the state[5][6].
Their analysis of the 2004 and 2007 enlargements is particularly instructive. Almost all the specific differentiations linked to membership disappeared after about ten to fifteen years of membership, which means that enlargement mainly generated a temporary differentiation rather than a permanent hierarchy between old and new members[7].
This precedent is important because it shows that differentiation and political equality are not necessarily contradictory. A temporary difference can allow for adaptation and then lead to a complete normalization of status.
The contemporary problem, however, is more complex. Future gaps may not only concern economic or administrative capabilities. They may also reflect enduring preferences on defense, taxation, foreign policy, or other sovereignty functions. In this case, simply waiting for states to « catch up » with a more integrated core may not match the nature of the disagreement.
6. Enlargement forces a distinction to be made between speed and destination
This distinction leads to the separation of two models of differentiated integration. In a multi-speed Europe, members follow the same trajectory but do not progress at the same pace. The gap is temporary. In a Europe that is permanently differentiated, the degree or scope of participation may remain different because the institutional preferences themselves diverge.
Schimmelfennig and Winzen precisely distinguish between multi-speed, multi-tier and multi-menu logics. In the first case, the differentiation is temporary. In the second, groups of states remain at different levels of integration for a long time. In the third, the composition of the participants varies according to the policies: a state may be deeply integrated in one area and less so in another[8].
This typology makes it possible to identify the specific challenge of the next enlargement. If the differences between the old and the new members mainly concern administrative or economic capacities, a multi-speed Europe may be sufficient. The new members gradually join the different policies when they meet the required conditions.
But if enlargement reveals permanently different conceptions of the sovereignty or purpose of the Union, the question becomes different. The objective can no longer necessarily be gradual standardisation. It is then necessary to know whether several legitimate degrees of integration can coexist in the same political order.
This question does not only concern the new members. Several current European differentiations have their origin in the preferences of old and relatively prosperous States. Constitutional heterogeneity is therefore not a characteristic of the candidates alone: it belongs to the general political structure of the Union.
7. An enlarged Union will have to define its common ground more clearly
Accepting sustainable differentiation does not mean that all policies should become optional. On the contrary, the more the perimeters of participation can vary, the more clearly the content of the common core must be defined.
The internal market is a first dimension of this pillar. Its functioning requires common rules, fair competition, freedom of movement and a sufficiently homogeneous legal order. Selective participation in which the advantages of the market can be enjoyed while refusing the obligations necessary for its functioning would quickly distort and weaken trust between members.
The rule of law, fundamental democratic principles and the effectiveness of European law constitute a second dimension. The Union cannot treat these elements as sectoral policies in which the States would choose whether or not to participate. They determine the conditions of mutual trust necessary for the functioning of the whole.
The common institutions are also an essential element. A differentiated Europe cannot function as an addition of independent organisations without a centre of coherence. The Commission, the Council, the Parliament and the Court must retain a role that makes it possible to articulate the various forms of cooperation with the common order.
Differentiation must therefore relate to the degree to which certain capacities are exercised together, not to the very existence of the European political system.
This principle is decisive in order to avoid the assimilation between differentiated integration and Europe à la carte. A Europe à la carte would theoretically allow each government to select the advantages of its choice. A viable differentiated Union, on the other hand, imposes a mandatory base and only allows variation in areas explicitly defined as differentiable.
8. Enlargement could transform accession itself into a more gradual process
The existence of a clear boundary between « member » and « non-member » could also become more difficult to maintain in an enlarged Union. Europe already has several forms of partial participation: some non-member states are deeply integrated into the internal market or Schengen, while some members do not participate in all EU policies.
Schimmelfennig and Winzen thus describe a European area characterised by several degrees of integration that partially cross the formal border of EU membership. Their analysis of regional differentiation shows that non-member states can be integrated into certain policies at very high levels, producing an architecture that is more complex than a simple separation between internal and external.[9]
In future enlargement processes, this reality could be mobilised more systematically. Candidates could be gradually integrated into certain programmes, markets, infrastructure or policies before their full accession. Such an approach would have several advantages: it would produce concrete benefits more quickly, make conditionality more progressive and facilitate the adaptation of administrations and economies.
However, it carries a risk: that of creating a permanent waiting status. If gradual integration becomes a substitute for membership rather than a path to it, candidates could be permanently associated with European obligations or policies without having all the political rights attached to membership.
The gradation must therefore remain politically legible. It must specify the rights obtained at each stage, the corresponding obligations and the conditions for moving to the next level.
9. Enlargement can strengthen the Europe of capabilities
Institutional transformation should not obscure the potential strategic benefits of enlargement. A larger Union would have a larger market, a larger territory, new resources, agricultural capacity, greater geographical depth and, in some cases, additional industrial or military skills.
Ukraine is a particularly significant example here. Its military experience, defence industry, technological potential and strategic position could transform several dimensions of European security. The Western Balkans also occupy a key geographical position for the continent’s infrastructure, energy, transport and security.
The European Parliament’s 2025 study highlights precisely these potential benefits by linking enlargement with a possible improvement in competitiveness, access to critical raw materials, sovereignty and European defence readiness[10].
But these additional resources will only become a European capability if they can be integrated into common policies. A larger defence industrial base has strategic value only if investments, standards and purchases can be coordinated. Energy or mineral resources only become sovereign if the infrastructure necessary for their exploitation and circulation exists.
Enlargement therefore potentially strengthens the European capacity building, but at the same time it increases the need for sufficiently integrated policies to convert these resources into power.
10. Capacity communities can provide an intermediate response
An enlarged Union could therefore operate around a more explicit distinction between policies common to all and communities of capacity bringing together only certain members. This architecture would make it possible to avoid two extreme solutions: imposing a politically impossible institutional uniformity or renouncing joint action when not all states wish to participate.
A community of capacity would differ from a mere diplomatic coalition in the degree to which it was institutionalized. Participants would agree to specific objectives, financial obligations, decision-making rules and control mechanisms. Entry would be voluntary, but obligations would become binding once participation was accepted.
Defence is an obvious area, but the logic can be applied to energy, critical technologies, infrastructure or certain financing. Enhanced cooperation could be justified when a group of States considers that the joint action produces a greater strategic advantage than the separate action.
This architecture would respond particularly well to a more heterogeneous Union. It would allow states that have both the political will and the material capacity to move forward without turning non-participants into adversaries or laggards.
However, it presupposes several safeguards: compatibility with common law, openness to other members, transparent distribution of costs, absence of discrimination and democratic control. Without these conditions, communities of capacity could gradually produce closed clubs and fragment the Union.
11. The major risk remains that of a multi-class Europe
Finally, sustainable differentiation encounters a major political limit: the risk of hierarchical membership. If the same states systematically belong to the most integrated policies and have the most significant capacities, while others remain permanently on the periphery, the Union can develop an informal structure of first and second categories.
This fear is directly identified in the literature on differentiated integration. Schimmelfennig and Winzen show that differentiation has historically facilitated integration and has generally been inclusive, but they also point to its limits in terms of solidarity and the risk of permanent divisions when the integration of certain central functions becomes dependent on distinct institutional trajectories[11].
A viable architecture must therefore ensure that differentiated communities remain open. The criteria for participation may be demanding, but they must be objective and functional. A state capable of meeting the necessary obligations must be able to join a cooperation if it so wishes.
The fundamental political status of the members must also remain equal. Participation in more policies cannot confer general superiority over the functioning of the Union. Differences must remain attached to the areas concerned.
The objective is thus to build several degrees of integration without building several degrees of belonging.
12. Enlargement forces us to rethink the very meaning of European convergence
The deepest question is ultimately neither budgetary nor institutional. It concerns the purpose of the European project. For several decades, differentiation could be understood as a temporary gap in a global process of convergence. Some states moved faster, others more slowly, but the idea of a common direction remained open enough for everyone to project their own conception of the project.
A Union of more than thirty States makes this presumption more difficult to maintain. Societies with different political histories, distinct strategic cultures and diverse conceptions of sovereignty can deeply share European belonging without necessarily wishing for the same level of centralisation in all policies.
This reality does not necessarily pose a threat to the Union. It only becomes problematic if the institutions continue to interpret each divergence as a temporary anomaly that needs to be corrected by further convergence.
Post-Europeanism provides another reading here. European success can be based not on the gradual standardisation of all preferences but on the ability to organise their coexistence politically. Integration retains all its value when it makes it possible to produce additional capacity, but its degree may vary according to policies and states.
Enlargement does not therefore mark the end of integration. On the contrary, it can lead to new and very ambitious forms of integration. But it does require a distinction to be made between the integration that is necessary for the existence of the Union and that which can legitimately remain differentiated.
Conclusion: Enlarging the Union now implies transforming its governance
Enlargement is one of the main strategic opportunities for contemporary Europe. It can strengthen the Union’s demographic and economic weight, stabilise its neighbourhood in the long term, increase its strategic depth, develop its access to certain resources and contribute to consolidating the European political order in the face of external coercion. It would therefore be simplistic to present it mainly as an institutional cost.
But precisely because enlargement has a geopolitical dimension, it cannot succeed if the Union is content to extend its institutional borders without adapting its political capacity. A Europe of more than thirty States will have to decide in a more heterogeneous environment, finance more demanding policies and at the same time maintain a sufficient level of trust to maintain the market, the law and common solidarity.
The model of progressive uniformity will then meet with increasing limits. Differentiation will still be necessary to absorb the differences in the capacity of the new members, as it has been in previous enlargements. But it could also become more sustainable in areas where political preferences do not converge.
The answer cannot be an à la carte Europe. On the contrary, the Union will have to define more clearly what constitutes its indivisible foundation: the internal market, the legal order, the institutions, fundamental values and the obligations necessary for the coherence of the whole. It is precisely because this pillar will remain robust that different degrees of integration can be accepted elsewhere.
Enlargement thus forces the Union to move from a quantitative question to an institutional one. The problem is no longer just how many states it can accommodate. It is to determine how these states can belong to the same political order without being obliged to participate in exactly the same way in all the dimensions of integration.
The success of enlargement will ultimately depend on the Union’s ability to meet two requirements together: to remain sufficiently united to constitute a coherent political space and to become sufficiently differentiated to remain governable in its diversity.
The Europe of more than thirty States will probably not be able to be simply the enlarged version of the Union of twenty-seven. It will have to become an architecture capable of organising several levels of participation in the long term without losing the unity that gives membership its value.
This is why enlargement is one of the decisive tests of post-Europeanism: it forces the Union to choose not only how far it wants to expand, but what political form can allow an enlarged Europe to remain able to act.
[1] European Council, 2023, Conclusions on Ukraine, enlargement and reforms, https://www.consilium.europa.eu/en/press/press-releases/2023/12/14/european-council-conclusions-on-ukraine-enlargement-and-reforms/
[2] European Council, 2024, Strategic Agenda 2024-2029, https://www.consilium.europa.eu/en/european-council/strategic-agenda-2024-2029/
[3] European Council, 2024, Strategic Agenda 2024-2029, https://www.consilium.europa.eu/en/european-council/strategic-agenda-2024-2029/
[4] Fernandes, M., & Saulnier, J. L., 2025, Towards renewed and beneficial EU enlargement, European Parliamentary Research Service, https://www.europarl.europa.eu/thinktank/en/document/EPRS_BRI(2025)765773
[5] Schimmelfennig, F., & Winzen, T., 2020, Ever Looser Union? Differentiated European Integration, Oxford University Press, https://academic.oup.com/book/36955
[6] Schimmelfennig, F., & Winzen, T., 2020, « The Choice for Differentiated Europe », https://academic.oup.com/book/36955/chapter-abstract/322249151
[7] Schimmelfennig, F., & Winzen, T., 2020, « Normalization: New Member States and Differentiated Integration », https://academic.oup.com/book/36955/chapter-abstract/322253263
[8] Schimmelfennig, F., & Winzen, T., 2020, « Concepts: Differentiated Integration and Its Modes », https://academic.oup.com/book/36955/chapter-abstract/322248900
[9] Schimmelfennig, F., & Winzen, T., 2020, « Regional Differentiation: Beyond the EU Member States », https://academic.oup.com/book/36955/chapter-abstract/322255160
[10] Fernandes, M., & Saulnier, J. L., 2025, Towards renewed and beneficial EU enlargement, European Parliamentary Research Service, https://www.europarl.europa.eu/thinktank/en/document/EPRS_BRI(2025)765773
[11] Schimmelfennig, F., & Winzen, T., 2020, Ever Looser Union? Differentiated European Integration, https://academic.oup.com/book/36955