EU Enlargement and Best Practices: Lessons from the Past for the Future?

https://doi.org/10.63189/HHAN3189

1. Introduction

The European Union has always been an evolving project – a vision of integration that has expanded from six founding members to a diverse union of 27 states. Every enlargement has been hailed as a triumph of European unity, but also as a complex political, economic, and institutional challenge. Since Russia’s war against Ukraine, the debate over EU enlargement has been revitalized, with Ukraine,^[1] Moldova,^[2] and Georgia^[3] formally applying for membership and the Western Balkans pressing for progress,^[4] which means: The path to accession is once again at the center of European politics,^[5] as candidate countries navigate a demanding process of legal alignment, economic adaptation, and democratic reform.

Given the EU’s decades of experience in managing accessions, one would assume that clear best practices have emerged – guiding principles that ensure smooth and successful integration. But have these lessons truly been crystallized into a set of best practices? Or is each enlargement too unique, too much shaped by geopolitical circumstances to yield a clear blueprint for the future?

2. First Enlargement Round: Denmark, UK, Ireland in 1973

2.1. Legal Foundations

From the outset, the treaties foresaw enlargement: Article 237 EEC^[6] (and equivalents in ECSC^[7] and Euratom^[8]) allowed any European state to apply. The process required unanimous Council approval and treaty adjustments, but no substantive criteria were defined – what counted as accession-worthy had to be defined in practice.

2.2. The Enlargement Process

The road to the first enlargement was anything but straightforward. The UK applied in 1961, seeking market access while preserving ties to the US. Denmark and Ireland followed, motivated by trade.^[9] France vetoed the UK twice, fearing US influence. Only after de Gaulle’s resignation in 1969 did talks progress. Norway, another early applicant, ultimately withdrew after a national referendum rejected EU membership in 1972.^[10]

The Hague Summit in 1969 opened the door for enlargement. At this point, the Member States had already made it abundantly clear that accession – though not yet codified in formal criteria – was conditional on one essential principle: full acceptance of the existing legal and political framework – alignment, not renegotiation.^[11]

Following the political green light at The Hague, negotiations moved swiftly. After more than a decade of stalled efforts, it took only two years – from 1970 to 1972 – to complete the talks.^[12] Yet, the process was not without complexity. The European Commission had expressed its concerns early on. It warned that enlargement must not weaken the cohesion or institutional efficiency of the Community. Economic and institutional divergence required long-term harmonization, and while no treaty reform was followed, the need for structural resilience was acknowledged. Politically, the accession of stable democracies was seen as a clear gain. The main focus remained on safeguarding the internal market, not political alignment – which was considered already present.^[13]

Negotiations themselves were mostly smooth. Denmark faced only few obstacles beyond transitional arrangements.^[14] Ireland, small in size and heavily dependent on the UK, negotiated safeguards for its more vulnerable sectors.^[15] The UK’s negotiations were more contentious, primarily due to questions about its financial contribution and adaptation to the Common Agricultural Policy. It was granted a six-year transition period for introducing the CAP,^[16] and a seven-year glide path for adjusting its financial obligations, including a safeguard clause.^[17] Fisheries were contentious but resolved with a ten-year compromise.^[18]

In sum: This first enlargement round was relatively unproblematic. Politically, the candidates shared the core democratic values of the original six. The acquis was still limited, focused mainly on economic integration, keeping adaptation manageable. The key was full acceptance of what had been achieved – plus commitment to go further. One size didn’t fit all, but fundamentals were aligned.

3. Second Enlargement Round: Greece, Spain and Portugal in 1981 and 1986

With Greece joining in 1981^[19] and Spain and Portugal in 1986,^[20] enlargement became a geopolitical tool. The goal was to stabilize young democracies and prevent backsliding into authoritarianism.^[21] Democracy, though not yet codified as a criterion, had become essential – already affirmed by the Birkelbach Report in 1962.^[22]

This enlargement saw more structured pre-accession steps. Greece had signed the Athens Agreement in 1961 to align its economy with the EC – frozen during its dictatorship but resumed post-1974.^[23] Spain and Portugal also signed preferential trade agreements in the early 1970s, laying the foundation for accession negotiations in 1978.^[24] Economically, the three countries were far from ideal candidates. Their economies were underdeveloped, their agricultural and industrial sectors structurally weak, and their GDPs well below the Community average. Moreover, Greece faced unresolved tensions with Turkey and a fragile economic foundation.^[25] But political momentum, especially in the Council, prevailed. Extensive transitional arrangements were agreed upon: Greece received a five-year transition period for most market rules, with free movement of workers delayed by seven years.^[26] Spain and Portugal were granted seven years for customs and free movement, and ten years for integration into the common agricultural and fisheries policies.^[27] The latter was given the scale of their respective sectors and the sensitivities they raised, especially with France and Italy.^[28]

This enlargement marked a shift: political values trumped economic readiness. Enlargement became more conditional and strategically framed. Though the Treaties stayed unchanged, the process evolved. It also triggered internal reform. The Single European Act (1986) introduced qualified majority voting, enhanced Parliament’s role, and made its assent mandatory for accessions – while paving the way for the single market.^[29] Enlargement appeared to act as a catalyst for integration.

4. Third Enlargement Round: Austria, Finland and Sweden in 1995

Before the next enlargement round, the Treaty on European Union (Maastricht Treaty), entering into force in November 1993, brought significant institutional and substantive changes. It formally established the European Union, introduced deeper integration via the three-pillar structure, and confirmed the legal pathway for accession under Article O, which maintained the unanimity requirement in the Council and the involvement of both Commission and Parliament.^[30] Additionally, the Copenhagen Criteria – set out by the European Council in June 1993 – were introduced: democracy, rule of law, human rights, market economy, and capacity to adopt the acquis.^[31] While designed with Eastern enlargement in mind, these criteria codified principles that had guided earlier rounds – and posed no challenge to Austria, Sweden, or Finland, given their political and economic maturity.

The end of the Cold War cleared the way for previously neutral countries to join. All three had already begun aligning economically with the EU through the EEA Agreement (1992).^[32] Applications for full membership were submitted between 1989 and 1991, negotiations began in 1993, and they proceeded quickly and smoothly. Austria secured a ten-year transitional period for transit traffic regulation and restrictions on property purchases by non-residents.^[33] Finland negotiated structural funds for its entire territory and protection for its food sector.^[34] Sweden negotiated a gradual phasing-out of reduced budget contributions.^[35] The Accession Treaty was signed on 24 June 1994, and after successful referenda (Norway again voted no), all three countries joined on 1 January 1995.^[36] This round proceeded in a straightforward manner – an economic boost without political or institutional strain.

5. Eastern Enlargement and its Continuation: Czech Republic, Estonia, Cyprus, Latvia, Lithuania, Hungary, Malta, Poland, Slovenia, and Slovakia in 2004, Bulgaria and Romania in 2007, Croatia in 2013

The Eastern enlargement – ten countries in 2004, two in 2007, and Croatia in 2013 – was the EU’s most ambitious expansion. Politically and economically, many candidates were far from ideal: with structural weaknesses and lower living standards. Perhaps precisely because of these challenges, this was arguably the most carefully prepared round. Building on the Copenhagen Criteria, the EU developed a detailed pre-accession framework: association agreements, chapter-by-chapter screening of the acquis, and continuous monitoring aimed at ensuring alignment before accession.^[37]

Legally, the EU adapted to match this scale. The Amsterdam Treaty (1999) formally enshrined core EU values as accession conditions – though they had long been applied in practice (see above).^[38] The Nice Treaty (2003) reformed decision-making for a Union of 25+,^[39] and the Lisbon Treaty (2009) linked enlargement directly to the EU’s values (Article 2 TEU).^[40]

Despite all this, transitional arrangements were essential. New members were gradually included in agricultural payments, and existing Member States restricted free movement of workers temporarily. Additional measures addressed land ownership (e.g. in Poland and the Baltics) and environmental standards (e.g. Poland granted extensions until 2017). The 2004 treaties alone contained around 265 transitional provisions, reflecting the complexity of integrating so many states at once.^[41] Still, strong political backing – especially from Germany, the UK, and Nordic countries – pushed enlargement forward as a means to stabilize post-socialist Europe.^[42] Romania and Bulgaria joined in 2007, with safeguard clauses for judicial reform and anti-corruption efforts.^[43] Croatia followed in 2013 after lengthy negotiations and under similarly detailed transitional arrangements across ten key policy areas.^[44]

6. Conclusion

Looking back at fifty years of repeated EU enlargement, one thing becomes clear: there is no universal formula. Every accession round has brought its own mix of political motivation, economic reality, and institutional adaptation. But if earlier rounds were shaped by shared democratic values and manageable economic gaps, the challenge today is of an entirely different order.

The acquis communautaire has grown exponentially – from around 10,000 pages in 1973 to well over 80,000 in 2004 and even more today.^[45] The EU is no longer a six-member economic club, but a highly integrated union of 27, with a far more diverse political and social fabric. Enlargement now means integrating not just new economies and legal systems, but vastly different political cultures into a system already under stress. What worked in 1973 – or even in 2004 – may no longer suffice. Enlargement has always been about managing complexity. But today, the EU must balance on a tightening rope – holding itself together while reaching further out. Today’s candidates are further away – economically, institutionally, and often politically, while the Union itself is facing its own cohesion challenges. Once, enlargement meant exporting stability. Today, it means importing fragility – while stability inside is no longer a given.

^[1] See European Commission, 2023, Ukraine 2023 Report of November 08, 2023.

^[2] See European Commission, 2024, Moldova.

^[3] See European Commission, 2024, Georgia.

^[4] See European Parliament, The Enlargement of the Union.

^[5] See European Council, 2023, The Granada declaration, where a consensus was reached during the European Council regarding further enlargement. Prior to the Granada Declaration, there was notable resistance from some Member States, highlighting a significant shift in consensus, see Bulgaria et al., 2022, Non-paper on the outcome of and follow-up to the Conference on the Future of Europe.

^[6] See Art. 237 of the Treaty establishing the European Economic Community of 25 March 1957, available at: https://eur-lex.europa.eu/legal-content/DE/TXT/PDF/?uri=CELEX:11957E/TXT, accessed: 20 March 2025.

^[7] See Art. 98 of the Treaty establishing the European Coal and Steel Community of 18 April 1951, available at: https://eur-lex.europa.eu/legal-content/DE/TXT/PDF/?uri=CELEX:11951K/TXT, accessed: 20 March 2025.

^[8] See Art. 205 of the Treaty establishing the European Atomic Energy Community (EURATOM) of 17 April 1957, available at: https://eur-lex.europa.eu/legal-content/EN/TXT/PDF/?uri=CELEX:11957A/TXT, accessed: 20 March 2025.

^[9] See Kaiser, W., Elvert, J., 2004, European Union Enlargement, A Comparative History, p. 194 f.

^[10] See Bjørklund, T., 1997, Old and New Patterns: The 'No' Majority in the 1972 and 1994 EC/EU Referendums in Norway, pp.143 ff.

^[11] See European Council, 1969, Final Communique of the Hague Conference, Nr 13.

^[12] See Arter, D., 1993, The Politics of European Integration in the Twentieth Century, p. 170 ff.; Brunn, G., 2002, Die Europäische Einigung, von 1945 bis heute, p. 183 ff.

^[13] See European Commission, 1967, Opinion on the Applications for Membership received from the United Kingdom, IreIand, Denmark and Norway, p. 11 and 12 f.; European Commission, 1969, Opinion submitted to the Council concerning the applications for membership from the United Kingdom, Ireland, Denmark and Norway, p. 11, 13, 27 and 36.

^[14] See Arter, D., 1993, The Politics of European Integration in the Twentieth Century, p. 163 f.; Kaiser, W., Elvert, J., 2004, European Union Enlargement, A Comparative History, p. 40.

^[15] See Arter, D., 1993, The Politics of European Integration in the Twentieth Century, p. 170 ff.; see for example Art. 88, Art. 93, Art. 137 and Declarations on the economic and industrial development of Ireland of the Treaty between the Member States of the European Communities and the Kingdom of Denmark, Ireland, the Kingdom of Norway and the United Kingdom of Great Britain and Northern Ireland concerning the accession of the Kingdom of Denmark, Ireland, the Kingdom of Norway and the United Kingdom of Great Britain and Northern Ireland to the European Economic Community and to the European Atomic Energy Community of 27 March 1972, available at: https://eur-lex.europa.eu/legal-content/EN/TXT/PDF/?uri=CELEX:11972B/TXT, accessed: 31 March 2025.

^[16] See ibid., Art. 52 and 54.

^[17] See ibid., Art. 127 ff.

^[18] See ibid., Art. 100 ff.

^[19] See Art. 2 of the Treaty between the Member States of the European Communities and the Hellenic Republic concerning the accession of the Hellenic Republic to the European Economic Community and to the European Atomic Energy Community of 28 May 1979, available at: https://eur-lex.europa.eu/legal-content/EN/TXT/PDF/?uri=CELEX:11979H/TXT, accessed: 07 April 2025.

^[20] See Art. 2 of the Treaty between the Member States of the European Communities and the Kingdom of Spain, the Portuguese Republic, concerning the accession of the Kingdom of Spain and the Portuguese Republic to the European Economic Community and to the European Atomic Energy Community of 12 June 1985, available at: https://eur-lex.europa.eu/legal-content/EN/TXT/PDF/?uri=CELEX:11985I/TXT, accessed: 07 April 2025.

^[21] See Kaiser, W., Elvert, J., 2004, European Union Enlargement, A Comparative History, p. 202.

^[22] See Birkelbach, W., 1961, Report on the political and institutional aspects of accession to or association with the Community.

^[23] See Association Agreement between the EEC and Greece from 9 July 1961, available at: https://www.cvce.eu/en/education/unit-content/-/unit/02bb76df-d066-4c08-a58a-d4686a3e68ff/1039c1de-c68c-43a2-a45a-66aef48c5c6f/Resources#ea36b530-f7ee-46f3-a26b-5dc4ea1a5508_en&overlay, accessed 01 April 2025.

^[24] See Agreement between the EEC and Spain from 29 June 1970, available at: https://www.cvce.eu/content/publication/2007/6/15/1ada9820-a7c1-4cac-bdaf-9429c544904a/publishable_en.pdf, accessed 01 April 2025; Agreement between the EEC and the Portuguese Republic from 22 July 1972, available at: https://webintra.cvce.eu/content/prepublication/2003/2/17/2240d5e4-085d-4b5d-a042-386b88de4be9/publishable_en.pdf, accessed: 01 April 2025.

^[25] See Mehlhausen, T., 2016, European Union Enlargement, Material Interests, Community Norms and Anomie, p. 72 ff.; Brunn, G., 2002, Die Europäische Einigung, von 1945 bis heute, p. 245 ff.

^[26] See Art. 25 ff. and Art. 45 ff. of the Treaty between the Members of the European Communities and the Hellenic Republic concerning the accession of the Hellenic Republic to the European Economic Community and the European Atomic Energy Community of 28 May 1979, available at: https://eur-lex.europa.eu/legal-content/EN/TXT/PDF/?uri=CELEX:11979H/TXT, accessed: 01 April 2025.

^[27] See Art. 30 ff., Art. 55 ff., Art. 68 ff., Art. 154 ff., Art. 189 ff., Art. 215 ff., Art. 233 ff. and Art. 346 ff. of the Treaty between the Members of the European Communities and the Kingdom of Spain, the Portuguese Republic concerning the accession of the Kingdom of Spain and the Portuguese Republic to the European Economic Community and the European Atomic Energy Community of 12 June 1985, available at: https://eur-lex.europa.eu/legal-content/EN/TXT/PDF/?uri=CELEX:11985I/TXT, accessed: 01 April 2025.

^[28] See Brunn, G., 2002, Die Europäische Einigung, von 1945 bis heute, p. 246 ff.

^[29] See Art. 6, 7, 8 and 13 of the Single European Act from 17 February 1986 (Luxembourg) / 28 February 1986 (The Hague), available at: https://eur-lex.europa.eu/legal-content/EN/TXT/PDF/?uri=CELEX:11986U/TXT, accessed: 01 April 2025.

^[30] See Art. O of the Maastricht Treaty: “Any European State may apply to become a Member of the Union. It shall address its application to the Council, which shall act unanimously after consulting the Commission and after receiving the assent of the European Parliament, which shall act by an absolute majority of its component members. The conditions of admission and the adjustments to the Treaties on which the Union is founded which such ad-mission entails shall be the subject of an agreement between the Member States and the applicant State. This agreement shall be submitted for ratification by all the Contracting States in accordance with their respective constitutional requirements.”, available at: https://eur-lex.europa.eu/legal-content/EN/TXT/PDF/?uri=CELEX:11992M/TXT, accessed: 07 April 2025.

^[31] The European Council used the following wording: “Membership requires that the candidate country has achieved stability of institutions guaranteeing democracy, the rule of law, human rights and respect for and protection of minorities, the existence of a functioning market economy as well as the capacity to cope with competitive pressure and market forces within the Union. Membership presupposes the candidate's ability to take on the obligations of membership including adherence to the aims of political, economic and monetary union. The Union's capacity to absorb new members, while maintaining the momentum of European integration, is also an important consideration in the general interest of both the Union and the candidate countries.“, see European Council, 1993, Conclusions of the Presidency, p. 13.

^[32] See Kaiser, W., Elvert, J., 2004, European Union Enlargement, A Comparative History, p. 202.

^[33] See Art. 70 of the Accession Act and Art. 11 Nr. 3 of Protocol No 9 of the Treaty between the Member States of the European Union and the Kingdom of Norway, the Republic of Austria, the Republic of Finland the Kingdom of Sweden, concerning the accession of the Kingdom of Norway, the Republic of Austria, the Republic of Finland and the Kingdom of Sweden to the European Union, 94/C 241/07, available at: https://eur-lex.europa.eu/legal-content/EN/TXT/?uri=CELEX:11994N/TXT, accessed: 03 April 2025.

^[34] See ibid., Art. 137 ff. of the Accession Act and Protocol No 6.

^[35] See ibid., Art. 134 and 137 f. of the Accession Act; Brunn, G., 2002, Die Europäische Einigung, von 1945 bis heute, p. 284 ff.

^[36] See Treaty between the Member States of the European Union and the Kingdom of Norway, the Re-public of Austria, the Republic of Finland the Kingdom of Sweden, concerning the accession of the Kingdom of Norway, the Republic of Austria, the Republic of Finland and the Kingdom of Swe-den to the European Union, 94/C 241/07, available at: https://eur-lex.europa.eu/legal-content/EN/TXT/?uri=CELEX:11994N/TXT, accessed: 03 April 2025.

^[37] See Lippert, B., 2011, The big easy? Growth, differentiation and dynamics of EU-enlargement policy, in: Diedrichs, U. et. al., 2011, Europe reloaded, Differentiation or Fusion, p. 256 f.

^[38] See amendment of Art. O through the Treaty of Amsterdam amending the Treaty on the European Union, the Treaties establishing the European Communities and certain related Acts, 97/C 340/01, available at: https://eur-lex.europa.eu/legal-content/EN/TXT/PDF/?uri=CELEX:11997D/TXT, accessed: 04 April 2025.

^[39] See amendments through the Treaty of Nice amending the Treaty on the European Union, the Treaties establishing the European Communities and certain related Acts, 2001/C 80/01, available at: https://eur-lex.europa.eu/legal-content/EN/TXT/PDF/?uri=CELEX:12001C/TXT, accessed: 04 April 2025. Specifics about the accession of the respective countries can be found in Vassiliou, G., 2007, The Accession Story, The EU from Fifteen to Twenty-Five Countries, p. 117 ff.

^[40] See Art. 49 TEU as we know it today.

^[41] See overview in Lippert, B., 2003, Erweiterungspolitik der Europäischen Union, in: Weidenfeld, W., Wessels, W., 2003, Jahrbuch der Europäischen Integration 2002/2003, p. 418 f. and 425; Ullrich, V., Rudhoff, F., 2004, Der Fischer Weltalmanach aktuell, Die EU-Erweiterung, p. 176 ff. and 180 ff.

^[42] See Brunn, G., 2002, Die Europäische Einigung, von 1945 bis heute, p. 294 ff.

^[43] See European Commission, 2006, Decision of 13 December 2006 establishing a mechanism for cooperation and verification of progress in Bulgaria to address specific benchmarks in the areas of judicial reform and the fight against corruption and organised crime (notified under document number C(2006) 6570), 2006/929/EC; European Commission, Decision of 13 December 2006 establishing a mechanism for cooperation and verification of progress in Romania to address specific benchmarks in the areas of judicial reform and the fight against corruption (notified under document number C(2006) 6569), 2006/928/EC; Lippert, B., 2007, Erweiterungspolitik der Europäischen Union, in: Weidenfeld, W., Wessels, W., 2007, Jahrbuch der Europäischen Integration 2007, p. 424.

^[44] See Lippert, B., 2012, Erweiterungspolitik der Europäischen Union, in: Weidenfeld, W., Wessels, W., 2012, Jahrbuch der Europäischen Integration 2012, p. 517 f.

^[45] See Lippert, B., 2011, The big easy? Growth, differentiation and dynamics of EU-enlargement policy, in: Diedrichs, U. et. al., 2011, Europe reloaded, Differentiation or Fusion, p. 257.