Enlargement has been crucial to boost European integration, including its territorial expansion, but also significant institutional and political changes. So far, the European Union (EU) completed seven rounds of enlargement and is currently invested in the accession process of ten aspiring members. These successive enlargements triggered dynamics of political widening and deepening that were not always consistently implemented, given the need to address challenges posed by the accession of new member states and by a more diverse Union. Through an overlook of past enlargements, this paper aims to assess if these have resulted in an increased “internal integration capacity” (Schimmelfenning, 2014; Borzel, Schimmelfenning and Dimitrova, 2017), in order to identify lessons learned, as well as challenges and opportunities in future enlargements.

For that purpose, the paper is organized into three main parts. The first provides the research design, the theoretical framework and a literature review on European enlargements, focusing on widening-deepening dynamics associated with these processes. The second part contextualizes the major rounds of enlargement, as well as the institutional and political changes induced as a response to challenges resulting from a territorial expanded and more diverse EU. In order to do so, we rely on an adaptation of the concept “internal integration capacity” to systematize assessments of the implications of enlargements in terms of public support, institutional deepening (the polity), and policy-making (including policy competences and decision-making). This analysis will be based on both primary – e.g., official documents and reports from the European Commission – and secondary sources. Furthermore, enlargements will be aggregated according to a geographical criterion – focusing on the first enlargement (1973), the Southern enlargement (1981; 1986), the Northern enlargement (1995) and the Eastern enlargement (2004; 2007).

The final part provides a critical assessment of the context in which future rounds of enlargement are being prepared, in order to identify opportunities and challenges to the EU. Conclusions from the data analysis in part two will be crucial to triangulate lessons from five decades of Enlargements with the complex matrix of challenges and opportunities ahead and their repercussions on the widening and deepening of European integration, thus contributing to a comprehensive and critical reflection upon the past, present and future of the EU. This is of crucial importance given the current tumultuous geopolitical environment at the regional and international levels, the EU’s internal institutional, political, socio-economic difficulties, including a rampant democratic backsliding.

The European Union (EU) operates on a political contract expecting efficiency, but respecting its member states (MS), ensuring fair staff representation within its institutions, which is crucial for its legitimacy and citizens' trust in its institutions. The European Commission (EC) employs approximately 32,000 permanent staff, guided by regulations requiring a balanced representation from all MS. Despite reforms, the system faces geographical imbalances, particularly disadvantaging the member states which joined the EU in 2004. Historically, Western Europe has dominated EU leadership roles, with Southern Europe recently gaining prominence, while Northern Europe maintains a fair representation relative to its population. In contrast, Central and East European (CEE) countries and smaller states such as Malta and Cyprus are underrepresented, often limited to less prominent EU agency roles.

Persistent under-representation impacts EU cohesion and legitimacy, potentially fostering Euroscepticism and democratic backsliding in affected states. This article investigates the progress of Slovakia, Poland, Malta, Cyprus, Estonia, and Czechia in embedding within EU structures over the past decade, focusing on staff representation within the EC. It examines whether the EU or the MS are responsible for ongoing imbalances. By analysing data from official action plans and structured interviews we systemize the factors. Challenges in institutional (including recruitment) processes, socio-economic, and cultural factors, are further explored together with practices from more successful new MS, such as Hungary or Slovenia.