The prolonged and increasingly uncertain trajectories of EU integration for candidate and potential candidate countries, including the Western Balkans, Turkey, and the Eastern Partnership states, have intensified debates about the future architecture of European integration. While political and institutional obstacles to enlargement are well documented, the legal feasibility of alternative forms of integration that fall short of full membership remains insufficiently explored. In particular, questions concerning the legal effects of non-EU countries' participation in EU regulatory regimes have received limited systematic attention. This paper addresses this gap by examining differentiated integration (DI) as a multilayered approach to EU enlargement beyond a linear accession model.

Existing accounts of Europeanization have traditionally conceptualised integration as a process oriented towards full membership as its political and legal endpoint. On the other hand, scholarship on differentiated integration highlights flexibility, variable geometry, and selective participation across policy domains, but is mostly studied in the context of relations among the EU member states. These approaches rarely engage with one another, especially in the legal scholarship. The paper brings them into dialogue by exploring whether and how the Europeanization of current candidates without full accession can be accommodated within the EU’s existing treaty framework.

Drawing on doctrinal legal analysis complemented by insights from political science, the paper examines the legal design and consequences of multi-circle models of EU participation. Particular attention is paid to how differentiated arrangements could affect decision-making procedures, accountability mechanisms, and the distribution of competences between EU institutions and participating non-member states. Rather than treating DI as a purely transitional or second-best solution, the paper analyses it as a mode of governance that produces distinct legal effects and strengthens incentives for reform and compliance in candidate countries. It argues that legally structured forms of differentiated participation may offer a pragmatic response to enlargement fatigue, while simultaneously raising fundamental questions about equality, representation, and the long-term integrity of the EU legal order if multi-circle solutions are employed for the candidates.

The prospects for further enlargement of the European Union (EU) have increased in recent years with several candidates making notable progress in their accession negotiations and the EU becoming more committed, at least rhetorically, to further enlargement than it has been for much of the last two decades. Assessments of the prospects for further enlargement focus on the state of play in accession negotiations and the shifting geopolitical context that has seen the Europea Council declare enlargement as a ‘geostrategic investment in peace, security, stability and prosperity’. Limited attention is being paid, however, to the prospects for an essential procedural requirement for enlargement being met, namely ratification of an accession treaty by all EU member states. This is despite the increased politicization of European integration and the evident party political and popular opposition to enlargement that exists in some member states. The extent to which opposition to enlargement has the potential to impact on the ratification of any accession treaty will depend, in part at least, on the ‘constitutional requirements’ each member state must fulfil to complete ratification. Of note here, is that in the case of France, the default procedure for ratification of accession treaties is now a national referendum. This paper provides an initial assessment of the ratification prospects for future EU accession treaties. It does so by establishing for each member state the ‘constitutional requirements’ for ratification and identifying where there is debate on alternative or supplementary procedures (e.g. a referendum) being used. The paper then assesses the extent of party political and poplar support for and opposition to enlargement both generally within each member state and with regard to specific candidate states before mapping these assessments onto the processes that it is expected member states will use to secure ratification. The paper concludes with an assessment of ratification prospects and so hitherto underexplored challenges ahead for the EU in securing further enlargement.

The EU enlargement has been considered one of the most successful instruments to foster peace and security on the European continent. Following the seven rounds, however, in the last decade the appetite to enlarge sharply decreased.

Russia’s full-scale invasion of Ukraine has been a watershed moment reinvigorating the process since 2022. Moscow’s aggression, Beijing’s geostrategic ambitions and the geopolitical winds blowing from Washington are challenges pushing the EU to strengthen its global posture. This great power competition, among others, is reflected in the re-emergence of enlargement discussions.

As both a geopolitical necessity and a merit-based process, the EU enlargement has attracted considerable attention from policymakers and academics. However, the existing scholarship has largely overlooked the informal aspects of the EU decision-making process in the context of enlargement. While most debates focus on the EU’s internal reform, conditionality, economic or security rationale, the role of informal groupings and discussions that drive the process has been understudied.

This paper tries to conceptualize enlargement within the wide spectrum of informal governance and argues in favour of including informality in the study of EU enlargement. Apart from attempting to explain how informality can be studied within enlargement domain, the study also showcases why the issue deserves particular attention both from scholars and policy makers, first and foremost by linking the case to intrinsic nature of the EU decision-making and the Union’s quest to become a global actor in light of great power competition.

The paper, particularly, aims at serving as a precursor for explaining the political dynamics within the EU capitals and Brussels behind the accession process of the candidates. Being a pioneer in this direction, the study specifically proposes how to theorize informal discourses in the enlargement domain. Apart from that, the paper proposes threefold explanatory model, examining how actors (leaders and policy-makers), networks (informal groupings) and discourses (public and closed-door narratives) shape the policymaking at the EU level vis-à-vis enlargement.

While the paper is theoretical in nature incorporating key trends and findings in enlargement and informality literature, it also uses empirical examples. The study represents a novelty in the emerging scholarship on the EU enlargement by bridging it to the equally relevant literature on informality in EU foreign policy. Consequently, the paper establishes a theoretical and conceptual framework for studying informal dimensions of the EU enlargement, bringing an added value to the enlargement and informality scholarship.

The article examines the shift of Kosovo-Serbia Dialogue from a diplomatic initiative into a conditionality benchmark in light of the EU’s enlargement framework. Existing studies discuss the political process of the Dialogue in terms of conflict resolution, regional stability and EU diplomacy, but do not delve into the transformation of diplomatic commitments into legally framed benchmarks for accession. Our contribution fills that research gap. It traces back the Dialogue’s transformation through key agreements (from the 2013 Brussels Agreement to the 2023 Ohrid Agreement) and demonstrates how initial political commitments have gradually transformed into measurable and legally binding benchmarks for accession. It also identifies and discusses the main challenges underlying the conditionality framework shedding light on ambiguous provisions, (differing) interpretations of normalisation of bilateral relations and the EU’s internal contestations on recognition of statehood in Kosovo. The research concludes that the Dialogue has become a unique example of a CFSP instrument evolving into formal legal conditionality shaping both countries accession process to the EU, but it also remains constrained by the political will of local actors and the EU itself.