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 EU’s traditional approach to enlargement has relied on the Copenhagen Criteria and acquis compliance as the measures to judge a candidate’s suitability for membership. However, these measures do not say much about the candidate’s actual readiness to assume both the burdens and privileges of membership. Instead, it is essentially assumed that competitiveness and convergence will more or less automatically follow from deeper integration.

This paper challenges that assumption. By following a different methodology, based on the concept of ‘Integration Maturity’, it suggests that a series of economic indicators, as well as the candidate’s economic performance over time, will better reflect a candidate’s preparedness to benefit from deeper integration, or, alternatively, suggest that more time should be spent achieving sufficient competitiveness and convergence.

To illustrate this point, this paper examines the cases of Slovenia and Croatia, who joined the EU in 2004 and 2013, respectively. Both had been part of Yugoslavia, and therefore had similar starting points for their economic transitions in the 1990s. Yet, while the former has prospered since accession, the latter is one of the worst performing members. What this paper suggests is, that this divergence was entirely predictable, based on the two countries’ economic performance on several indicators during the years leading to accession. This finding has implications for the way future accessions should be assessed, especially as the EU has recently decided to open accession talks with several more countries, and since politics may well trump economic considerations.

The process of democratic consolidation in Spain after the Franco dictatorship was marked, in its external dimension, to a large extent by the insertion of the Iberian country into Western international organizations. The entry into NATO, forced in the spring of 1982 by a center-right parliamentary majority against a social majority (about 75% of public opinion was against entry into NATO), was questioned by the new government led by Felipe González even before it came to power at the end of the same year.
Recently published studies have analyzed the proactive and essential facilitating role that the Federal Republic of Germany, under the leadership of Chancellor Helmut Kohl, played between 1982 and 1985 to help Spain overcome all the obstacles on its path into the European Communities. Against great reluctance on the part of France and Italy, Kohl imposed the accession of Spain and Portugal, conditioning the increase in the community budget, from which the Mediterranean members benefited first, and the UK rebate on the entry of the Iberian countries (Pact of Stuttgart).
The purpose of this paper is to shed light on another relationship of conditionality drawn up in Bonn and progressively implemented in Spanish-German bilateral relations: a link between the long-awaited entry of the European Communities and remaining in NATO. At the height of the Second Cold War, Bonn understood it as a matter of its national security for Spain to refrain from leaving NATO, since it would have meant a weakening of the unity of the West in the face of a USSR that, after the invasion of Afghanistan, showed itself more aggressive and hostile and less predictable. For Germany as a divided nation with a direct border with the Soviet bloc, and with the prospect of a nuclear war fought on its soil, Spain within NATO was important enough to make Felipe González understand that the Chancellor would only support Spain's full and comprehensive integration into the West, which necessarily involved double membership in Brussels: in the European Communities and in NATO. Based on unpublished diplomatic and political sources from German archives, the paper portrays in detail the strategy developed by Kohl to achieve this goal and its consequent application in bilateral relations with Madrid and within the Atlantic Alliance itself. González finally became a fervent defender of “NATO yes” and managed to drag his party and also the Spanish people.