EU Membership and the Development of the European Union: From Six Founding States to Twenty-Seven Members

EU Membership and the Development of the European Union: From Six Founding States to Twenty-Seven Members

Introduction

The development of the European Union from six founding states into a political and economic union of twenty-seven Member States is one of the most significant transformations in modern European history. What began as a limited project of economic cooperation among countries seeking to prevent another major European war gradually became a supranational legal and political order encompassing much of the European continent.

The original European Communities were established by six Western European states: Belgium, France, the Federal Republic of Germany, Italy, Luxembourg, and the Netherlands. Their cooperation was initially concentrated on strategic industries, trade, economic integration, and the creation of common institutions. Membership subsequently expanded through successive accessions, reflecting the changing political circumstances of Europe, the consolidation of democracy in Southern Europe, the end of the Cold War, the reunification of the continent, and the growing attractiveness of the European economic and legal model.

Enlargement was not simply a matter of increasing the number of participating countries. Each accession altered the Union’s political composition, economic geography, institutional balance, and legal responsibilities. The admission of Greece, Spain, and Portugal was closely connected with the democratic transformation of Southern Europe. The accession of Austria, Finland, and Sweden strengthened the Union’s Northern and Central European dimension. The enlargement of 2004 brought ten states into the Union, including eight from Central and Eastern Europe, marking a historic step towards overcoming the political division created by the Cold War. Bulgaria and Romania joined in 2007, followed by Croatia in 2013.

The process has not been irreversible in every respect. The United Kingdom, which joined in 1973, withdrew from the Union on 31 January 2020 following the Brexit referendum and subsequent negotiations. Its departure reduced the membership from twenty-eight to twenty-seven states.

Today, EU membership is governed by a combination of political eligibility, legal requirements, economic conditions, institutional preparation, and unanimous agreement among the existing Member States. The process is based principally on Article 49 of the Treaty on European Union and the membership criteria developed by the European Council.

Understanding the history of enlargement is essential to understanding the character of the Union itself. The European Union is not merely an organisation whose membership has expanded. It is a legal and political system that has repeatedly adapted its institutions, policies, and constitutional arrangements in response to the admission of new states.

1. The Six Founding States: The Origins of European Membership

1.1 The political context after the Second World War

The origins of European integration lie in the political and economic consequences of the Second World War.

Much of Europe had experienced destruction, displacement, economic collapse, and profound political instability. The relationship between France and Germany was particularly important. Repeated conflict between the two countries had contributed to European instability for generations, and the devastation of the Second World War created a strong political incentive to establish a more durable system of cooperation.

The emerging Cold War also influenced the development of integration. Western European states faced the strategic challenge of rebuilding their economies, maintaining political stability, and cooperating in the face of Soviet influence in Eastern Europe.

The idea of integration was based on a distinctive proposition: peace could be strengthened by creating institutions that made states economically interdependent and required them to manage important common interests through shared legal and political structures.

The Schuman Declaration of 9 May 1950 proposed placing French and German coal and steel production under a common High Authority. These industries were strategically important because they were essential to both economic reconstruction and military production.

The proposal led to the Treaty of Paris, signed in 1951, which established the European Coal and Steel Community (ECSC). It entered into force in 1952.

1.2 The original six

The six founding states of the European Communities were:

• Belgium;
• France;
• the Federal Republic of Germany;
• Italy;
• Luxembourg; and
• the Netherlands.

These states signed the Treaty of Paris establishing the ECSC and later signed the Treaties of Rome on 25 March 1957, establishing the European Economic Community (EEC) and the European Atomic Energy Community (Euratom).

The EEC Treaty entered into force on 1 January 1958. It established a common market and provided the foundations for the gradual removal of barriers to trade, the development of common policies, and the movement towards closer economic integration.

The six founding states were not merely participants in an international agreement. They established a new institutional system in which common bodies could adopt decisions with legal consequences for the participating states and, increasingly, for individuals and businesses.

Their original cooperation was limited in scope compared with the modern Union. Nevertheless, the legal and institutional foundations created by these states made subsequent enlargement possible.

1.3 Why the founding membership mattered

The composition of the original six reflected a particular historical and political environment.

They were Western European democracies seeking economic reconstruction, political stability, and closer cooperation. Their membership did not encompass the entire continent. Much of Central and Eastern Europe was under communist rule and politically aligned with the Soviet Union.

The original Communities therefore developed within a divided Europe. Their expansion would eventually become closely connected with the transformation of that division.

The founding states also established an important principle that continues to shape membership: participation in European integration requires acceptance of common rules and institutions. Membership is not simply a diplomatic alliance. It entails legal obligations, participation in common decision-making, and acceptance of a developing body of European law.

2. The First Enlargement: Denmark, Ireland, and the United Kingdom (1973)

2.1 The enlargement towards Northern and Western Europe

The first enlargement took place on 1 January 1973, when Denmark, Ireland, and the United Kingdom joined the European Communities.

Their accession increased the membership from six to nine states and marked the first major geographical expansion of the Communities.

The enlargement was the result of several years of negotiations. The United Kingdom had initially remained outside the EEC, partly because of its preference for a broader free-trade arrangement rather than the deeper economic integration envisaged by the founding states.

In 1960, the United Kingdom helped establish the European Free Trade Association (EFTA), alongside several other European countries. EFTA offered an alternative model of economic cooperation based primarily on free trade rather than a customs union and common institutions.

However, the growing economic importance of the EEC gradually changed the British position. The expansion of the common market and the success of the Communities encouraged the United Kingdom to seek membership.

Its first application in 1961 was unsuccessful. A second application in 1967 also failed after opposition from France, whose President Charles de Gaulle questioned the United Kingdom’s political and economic orientation and its relationship with the United States.

Following changes in French leadership, negotiations eventually progressed. The accession treaties were signed in 1972 and entered into force on 1 January 1973.

2.2 The United Kingdom’s membership

The United Kingdom’s accession was particularly important because of the country’s economic size, international influence, and historical trading relationships.

Its membership strengthened the Communities’ economic position and introduced a state with a different political tradition of parliamentary sovereignty, common-law institutions, and global commercial connections.

British participation also influenced the subsequent development of European integration. The United Kingdom supported certain aspects of economic liberalisation and the internal market while remaining more cautious about political centralisation and deeper institutional integration.

Its relationship with the European Communities was therefore characterised by both substantial participation and persistent constitutional debate.

2.3 Ireland and Denmark

Ireland’s accession was closely connected with its economic relationship with the United Kingdom. At the time, a substantial proportion of Irish trade was linked to the British market. Membership offered the opportunity to diversify economic relationships and participate directly in the development of European economic policy.

Denmark’s accession reflected its economic interests and its close connections with Western European markets. Its membership also demonstrated that the Communities could expand beyond the original continental group.

The first enlargement established a pattern that would recur throughout the history of the Union: accession was shaped by a combination of economic considerations, domestic political debate, and the wider strategic interests of European integration.

2.4 Norway’s rejection of membership

Norway also participated in the negotiations but did not become a member.

A referendum held in September 1972 rejected accession. Consequently, Norway did not join the Communities when the first enlargement entered into force.

This outcome illustrates an important feature of European membership: even where governments negotiate and sign an accession treaty, domestic constitutional requirements and political approval remain essential.

Norway subsequently developed close economic relations with the Union through the European Economic Area, but it has remained outside the EU.

3. The Second Enlargement: Greece (1981)

3.1 The return to democracy

Greece became a member of the European Communities on 1 January 1981, increasing membership to ten states.

Its accession was closely connected with the restoration of democracy following the collapse of military rule in 1974.

The military dictatorship that had governed Greece from 1967 to 1974 had interrupted the country’s democratic constitutional development. Following the restoration of civilian government, Greece sought closer integration with Western European institutions.

Membership was viewed not only as an economic opportunity but also as a means of consolidating democratic institutions and strengthening the country’s political orientation towards Europe.

3.2 The application and negotiations

Greece submitted its application for membership in 1975. Negotiations began in 1976 and resulted in an accession treaty signed in 1979.

The accession process required Greece to adapt its legal and economic systems to the obligations of Community membership.

Its economy differed considerably from those of several existing Member States, particularly in terms of income levels, industrial structure, and agricultural dependence. These differences created challenges for the gradual integration of the Greek economy into the common market and common policies.

Nevertheless, the political significance of accession was substantial.

3.3 The democratic significance of Greek accession

Greece’s membership demonstrated that European integration could serve as a framework for democratic consolidation.

The Communities were increasingly understood as a political community founded not only on economic cooperation but also on democratic government and respect for common legal principles.

The accession of Greece also helped establish the broader political context for the subsequent enlargement towards Portugal and Spain, both of which had experienced authoritarian regimes before their democratic transitions.

The relationship between democracy and membership would later become more formally expressed through the Copenhagen criteria.

4. The Third Enlargement: Spain and Portugal (1986)

4.1 Democratic transition in the Iberian Peninsula

Spain and Portugal joined the European Communities on 1 January 1986, bringing membership to twelve states.

Their accession followed the end of authoritarian government in both countries.

Portugal underwent a political transformation following the Carnation Revolution of April 1974, which brought an end to the Estado Novo regime. Spain’s transition began following the death of Francisco Franco in November 1975 and the subsequent establishment of democratic constitutional institutions.

Both countries viewed membership as a means of consolidating democratic government, modernising their economies, and strengthening their place within Western Europe.

4.2 The negotiations and accession

Portugal applied for membership in 1977, followed by Spain in the same year. Negotiations were lengthy and involved difficult questions concerning agriculture, fisheries, industrial competition, and the adaptation of national economic systems to the common market.

The existing Member States were concerned about the effects of admitting two countries whose economies differed significantly from the Community average.

Spain’s agricultural sector and fishing industry were particularly important in the negotiations. Portugal also required transitional arrangements to facilitate economic adaptation.

The accession treaties were signed in 1985 and entered into force on 1 January 1986.

4.3 The political and economic consequences

The accession of Spain and Portugal was a major step in the democratic consolidation of Southern Europe.

It extended the Communities’ geographical reach towards the Atlantic and Mediterranean and strengthened their cultural, economic, and political diversity.

It also demonstrated that enlargement could function as an instrument of political transformation. Membership offered access to economic opportunities and common institutions, but it also required the new states to consolidate democratic systems and comply with Community law.

The enlargement increased the importance of regional development policies. Economic disparities between Member States became more pronounced, strengthening the political case for financial instruments designed to support less-developed regions.

This contributed to the development of cohesion policy and the broader debate about solidarity within an expanding European economic area.

5. The Fourth Enlargement: Austria, Finland, and Sweden (1995)

5.1 The changing European environment after the Cold War

The fourth enlargement took place on 1 January 1995, when Austria, Finland, and Sweden joined the European Union.

By this time, the geopolitical circumstances of Europe had changed dramatically.

The fall of the Berlin Wall in 1989, the collapse of communist regimes across Central and Eastern Europe, and the dissolution of the Soviet Union had transformed the European political landscape.

The Maastricht Treaty had entered into force on 1 November 1993, establishing the European Union and creating a new framework for political and economic integration.

The accession of Austria, Finland, and Sweden therefore took place within a Union that was no longer simply the European Economic Community. It was an increasingly integrated political and legal structure with ambitions extending beyond the common market.

5.2 The characteristics of the new Member States

Austria, Finland, and Sweden had developed close economic relationships with the European Communities through their participation in EFTA and the European Economic Area.

Their economies were highly developed, and their legal and administrative systems were generally well aligned with the standards required for membership.

However, their constitutional and political traditions raised questions about neutrality, foreign policy, and participation in deeper European integration.

Austria had a constitutional commitment to permanent neutrality. Finland had historically maintained a particular security and foreign-policy position during the Cold War. Sweden also followed a policy of military non-alignment.

Membership required these countries to consider how their established policies would interact with the Union’s developing political and external responsibilities.

5.3 The Norwegian referendum

Norway again negotiated accession but rejected membership in a referendum held in November 1994.

As in 1972, Norwegian voters rejected joining the Union. The result demonstrated that economic integration and close cooperation did not necessarily produce political acceptance of full membership.

Austria, Finland, and Sweden joined on 1 January 1995, increasing the Union’s membership to fifteen.

5.4 The significance of the 1995 enlargement

The accession of these three countries strengthened the Union’s Northern European dimension and expanded its economic and institutional capacity.

It also brought states with established democratic traditions, advanced public administrations, and developed social and environmental policies into the Union.

Their participation contributed to debates concerning transparency, environmental protection, public administration, and the future direction of European integration.

The 1995 enlargement marked the last accession before the major expansion towards Central and Eastern Europe.

6. The Fifth Enlargement: The Historic Expansion of 2004

6.1 The reunification of a divided continent

The enlargement of 1 May 2004 was the largest in the history of European integration, both in the number of countries admitted and in its political significance.

Ten states joined the European Union:

• Cyprus;
• the Czech Republic, now commonly referred to as Czechia;
• Estonia;
• Hungary;
• Latvia;
• Lithuania;
• Malta;
• Poland;
• Slovakia; and
• Slovenia.

The enlargement increased membership from fifteen to twenty-five states.

Eight of the ten new members had previously been part of the communist political and economic sphere of Central and Eastern Europe. Cyprus and Malta represented the Mediterranean dimension of the enlargement.

The admission of these states was widely regarded as a historic step towards overcoming the division of Europe created after the Second World War.

For decades, the Iron Curtain had separated Western European democracies from states governed by communist regimes and closely connected with the Soviet Union. The political transformations of 1989–1991 created the conditions for a new relationship between the two parts of the continent.

The European Union offered the prospect of political stability, economic modernisation, democratic consolidation, and participation in a common legal and economic order.

Enlargement was therefore both a legal process and a broader historical project.

6.2 The transformation of Central and Eastern Europe

The Central and Eastern European applicant states faced substantial challenges in preparing for membership.

Their political institutions had to be adapted to democratic government, independent judicial systems, constitutional accountability, and the protection of individual rights.

Their economies were undergoing the transition from centrally planned systems to market-based economies. This involved privatisation, the development of competition, financial-sector reform, restructuring of state enterprises, and the creation of institutions capable of enforcing commercial and regulatory law.

The legal transformation was equally extensive. Membership required the adoption and effective implementation of the acquis communautaire: the body of existing EU law, including the Treaties, legislation, judicial principles, international agreements, and other obligations forming part of the Union’s legal order.

The applicant states were not simply required to declare that they accepted European rules. They had to demonstrate that their administrative institutions, courts, regulatory authorities, and public services could apply those rules in practice.

This distinction between formal adoption and effective implementation became one of the central themes of enlargement policy.

6.3 The Copenhagen European Council and the membership criteria

The political foundations of the enlargement were established at the European Council meeting in Copenhagen in June 1993.

The European Council identified the principal conditions that countries seeking membership would have to satisfy. These became known as the Copenhagen criteria.

They require applicant states to demonstrate:

Political stability and democratic institutions. A candidate must possess stable institutions guaranteeing democracy, the rule of law, human rights, and respect for and protection of minorities.

A functioning market economy. The country must have an economic system capable of operating within the Union’s internal market and coping with competitive pressure and market forces.

The ability to assume the obligations of membership. The candidate must be capable of adopting and implementing the acquis, including the objectives of political, economic, and monetary union.

The European Council also recognised the importance of the Union’s own capacity to absorb new members while maintaining the momentum of European integration.

The Copenhagen criteria were politically decisive because they established a common framework for assessing candidates. They also made clear that geographical eligibility alone was insufficient.

The enlargement process would depend on measurable institutional, economic, and legal preparation.

6.4 The accession negotiations

The European Council meeting in Luxembourg in December 1997 initiated the enlargement process with a group of Central and Eastern European applicant states and Cyprus. Negotiations proceeded in stages, and the process was later expanded to include additional candidates.

The negotiations involved a detailed examination of the acquis, organised into policy areas and chapters. The candidates were required to demonstrate their ability to adopt, apply, and enforce the relevant Union rules.

The negotiations also addressed transitional arrangements. Some existing Member States feared that immediate and unrestricted application of all internal-market rules could create economic or administrative difficulties. The new states, in turn, sought transitional periods in sensitive sectors.

The accession process therefore combined legal harmonisation, economic preparation, political negotiation, and institutional assessment.

The Accession Treaty was signed in Athens on 16 April 2003. Following ratification, the ten states joined on 1 May 2004.

6.5 The constitutional significance of the 2004 enlargement

The 2004 enlargement changed the character of the Union in several ways.

First, it extended the Union’s geographical and political reach across much of Central Europe and the Baltic region.

Second, it introduced significant differences in economic development, administrative capacity, and institutional experience. The Union had to develop policies capable of addressing greater economic diversity.

Third, the enlargement increased the number of languages, political traditions, national interests, and historical perspectives represented in the institutions.

Fourth, it strengthened the Union’s role as a framework for democratic and legal transformation.

The enlargement also increased the need for institutional reform. A system designed for six or fifteen states could not necessarily operate with the same efficiency in a Union of twenty-five or more members. This institutional pressure formed part of the wider context in which the Constitutional Treaty and, later, the Treaty of Lisbon were developed.

7. The Sixth Enlargement: Bulgaria and Romania (2007)

7.1 The accession of Bulgaria and Romania

Bulgaria and Romania joined the European Union on 1 January 2007, bringing membership to twenty-seven states.

Their accession was part of the broader post-Cold War enlargement towards Central and Eastern Europe. Both countries had undergone major political and economic transformations following the collapse of communist government.

Bulgaria’s transition began in 1989, following the end of the communist regime. Romania experienced a more violent political transition in December 1989, which resulted in the fall of Nicolae Ceaușescu’s government.

Both states adopted democratic constitutions, introduced market-oriented economic reforms, and pursued integration with European and transatlantic institutions.

They applied for EU membership in 1995 and began accession negotiations in 2000.

7.2 The conditions for accession

The accession process required extensive reforms in both countries.

Important areas included judicial independence, the fight against corruption, public administration, competition policy, agricultural regulation, environmental protection, border management, and the capacity to implement EU law.

Although both countries made substantial progress, concerns remained about the effectiveness of judicial and administrative reforms.

The Union therefore used transitional arrangements and additional monitoring mechanisms to address areas requiring further progress.

The Accession Treaty was signed in April 2005. Bulgaria and Romania subsequently joined on 1 January 2007.

Their accession demonstrated that enlargement was not limited to the states admitted in 2004. The Union continued to extend membership to countries that had emerged from communist systems and were undertaking institutional and economic transformation.

7.3 The Cooperation and Verification Mechanism

A distinctive feature of the 2007 enlargement was the Cooperation and Verification Mechanism (CVM).

The mechanism was established by the European Commission to monitor progress in particular areas of judicial reform and the fight against corruption in Bulgaria and Romania.

For Bulgaria, the benchmarks included judicial reform, the investigation and prosecution of high-level corruption, and the prevention and fight against corruption in public institutions.

For Romania, the benchmarks focused on judicial reform, integrity, and the fight against high-level corruption.

The mechanism reflected the Union’s concern that formal transposition of European standards did not always guarantee effective institutional performance.

It also raised an important constitutional question: how could the Union ensure that membership obligations were fulfilled after accession, when the candidate state had already acquired full Member State status?

The CVM was not a general suspension of membership rights. It was a monitoring framework designed to encourage and assess progress in specific areas.

The Commission concluded the mechanism for Bulgaria and Romania in 2023, following its assessment that the relevant benchmarks had been satisfactorily addressed. The experience nevertheless demonstrated the importance of institutional capacity and the rule of law in the enlargement process. Enlargement and Eastern Neighbourhood+2

8. The Seventh Enlargement: Croatia (2013)

8.1 The Western Balkans and European integration

Croatia joined the European Union on 1 July 2013, becoming the twenty-eighth Member State at the time.

Its accession was particularly significant because it represented the first enlargement involving a country that had emerged from the dissolution of Yugoslavia.

The political and economic transformation of the Western Balkans was shaped by the conflicts of the 1990s, the creation of new states, institutional reconstruction, and the need to establish stable democratic systems.

Croatia declared independence in 1991 and subsequently experienced armed conflict and significant economic disruption. Its application for EU membership was submitted in 2003.

8.2 Accession negotiations and legal reforms

Croatia’s negotiations required extensive reforms concerning the judiciary, public administration, competition, public procurement, fundamental rights, and cooperation with international criminal justice institutions.

The Union paid particular attention to the effectiveness of judicial institutions and the ability of the Croatian administration to implement European legislation.

The negotiations were concluded in 2011, and the Accession Treaty was signed on 9 December of that year. Following ratification, Croatia joined on 1 July 2013.

8.3 The significance of Croatia’s accession

Croatia’s accession demonstrated that the enlargement process remained open to countries of the Western Balkans, provided that they met the required conditions.

It also illustrated the increasingly detailed nature of accession negotiations. The Union’s experience with previous enlargements had led to greater emphasis on the rule of law, judicial independence, administrative capacity, and the practical enforcement of legal standards.

Croatia remained the most recent country to join the European Union as of 1 November 2026.

9. The Withdrawal of the United Kingdom: From Twenty-Eight to Twenty-Seven Members

9.1 The referendum of 2016

The United Kingdom’s departure from the European Union was an unprecedented development in the history of European integration.

The United Kingdom had joined the European Communities on 1 January 1973 and had participated in the development of the internal market, common commercial policy, institutional arrangements, and numerous areas of European cooperation for nearly five decades.

Nevertheless, British membership had been accompanied by recurring political debate about sovereignty, parliamentary authority, economic integration, immigration, and the extent of European institutional power.

On 23 June 2016, the United Kingdom held a referendum on EU membership. A majority of voters supported leaving the Union.

The referendum did not itself terminate the United Kingdom’s membership. Withdrawal required the formal procedure established by Article 50 TEU.

9.2 The Article 50 notification

On 29 March 2017, the United Kingdom formally notified the European Council of its intention to withdraw.

This activated Article 50 TEU, which provides for negotiations concerning the arrangements for withdrawal and the framework of the future relationship between the Union and the departing state.

The negotiations involved complex questions, including:

• citizens’ rights;
• the financial settlement;
• the status of the Irish border;
• the future relationship between Northern Ireland and the Republic of Ireland;
• the rights and obligations arising from existing EU programmes; and
• the legal arrangements governing the transition out of membership.

The process also demonstrated the importance of the Union’s institutions in managing withdrawal. The Commission negotiated on behalf of the Union under the political direction established by the European Council and the negotiating mandate adopted by the Council.

9.3 Withdrawal on 31 January 2020

The United Kingdom formally left the European Union on 31 January 2020.

A transition period followed, during which EU law continued to apply to the United Kingdom under the Withdrawal Agreement, subject to the specific provisions of that agreement.

The transition period ended on 31 December 2020. The subsequent relationship between the United Kingdom and the Union was governed principally by the Trade and Cooperation Agreement, alongside the Withdrawal Agreement and other arrangements.

The United Kingdom consequently became a third country for the purposes of EU law, although it remained subject to certain obligations arising from the withdrawal arrangements and its subsequent agreements with the Union.

9.4 The constitutional significance of Brexit

Brexit demonstrated that EU membership, although intended to support long-term integration, is not legally irrevocable.

It also exposed the practical complexity of withdrawal from a deeply integrated legal and economic system.

Membership affects citizens’ rights, trade, movement, regulation, judicial cooperation, financial obligations, and the application of common legal standards. Leaving the Union therefore required more than terminating participation in its political institutions.

The United Kingdom’s withdrawal reduced the number of Member States from twenty-eight to twenty-seven. It also changed the Union’s economic and political balance, including the relative weight of different Member States in the Council and the composition of the European Parliament.

Brexit remains a significant constitutional example of the operation of Article 50 TEU and the relationship between national democratic decision-making and the legal obligations arising from EU membership. Consilium+2

10. The European Union of Twenty-Seven Member States

10.1 The current membership

As of 1 November 2026, the European Union consists of twenty-seven Member States.

They are:

Member State	Year of accession
Belgium	1958
France	1958
Germany	1958
Italy	1958
Luxembourg	1958
Netherlands	1958
Denmark	1973
Ireland	1973
Greece	1981
Portugal	1986
Spain	1986
Austria	1995
Finland	1995
Sweden	1995
Cyprus	2004
Czechia	2004
Estonia	2004
Hungary	2004
Latvia	2004
Lithuania	2004
Malta	2004
Poland	2004
Slovakia	2004
Slovenia	2004
Bulgaria	2007
Romania	2007
Croatia	2013

The founding states are listed with 1958 as the year in which the Treaties of Rome establishing the EEC and Euratom entered into force. Their earlier participation in the ECSC dates from 1952.

The current membership is the result of seven enlargement rounds, followed by the United Kingdom’s withdrawal. Consilium+2

10.2 Membership does not mean identical participation in every EU policy

Although all Member States are bound by the Treaties and participate in the Union’s institutional system, they do not necessarily participate in every policy arrangement in exactly the same way.

The euro area, Schengen area, and certain forms of enhanced cooperation illustrate this differentiated structure.

For example, not every EU Member State uses the euro. Some states have not yet fulfilled the conditions for adopting the common currency, while Denmark has a specific treaty-based opt-out.

Similarly, the Schengen area includes EU Member States as well as certain non-EU countries associated with the system. Participation in Schengen and membership of the Union are therefore legally distinct concepts.

Differentiated integration allows Member States to participate in certain common arrangements at different stages or under different legal conditions. It does not, however, remove their obligations under the general EU treaty framework.

This distinction is essential when examining the legal position of individual Member States. Membership of the Union does not automatically establish membership of the euro area, the Schengen area, or every other form of enhanced cooperation.

10.3 The diversity of the modern Union

The twenty-seven Member States differ in population, geographical position, economic structure, constitutional organisation, legal traditions, and historical experience.

The Union includes large and small states, parliamentary and semi-presidential systems, civil-law and common-law traditions, highly industrialised economies, and economies with different levels of development.

This diversity is reflected in the institutions. The Council represents national governments; the European Parliament represents Union citizens; and the Commission is required to promote the general interest of the Union.

The legal system must accommodate different national constitutional traditions while maintaining the effectiveness and uniform application of Union law.

The principle of equality of Member States before the Treaties coexists with institutional arrangements that take population into account, particularly in the composition of the European Parliament and qualified majority voting.

The Union is therefore neither a simple association of equal sovereign states in which every decision requires unanimity nor a federal state in which national governments have lost their constitutional identity. It is a distinctive legal order combining supranational authority with the continuing participation of its Member States.

11. The Legal Basis for Membership: Article 49 TEU

11.1 Eligibility for accession

The principal legal basis for joining the European Union is Article 49 TEU.

It provides that any European state which respects the values referred to in Article 2 TEU and is committed to promoting them may apply to become a member of the Union.

The reference to a European state establishes a geographical and political condition. However, European location alone is not sufficient.

The applicant must also respect the Union’s fundamental values, including human dignity, freedom, democracy, equality, the rule of law, and human rights, including the rights of persons belonging to minorities.

The applicant must demonstrate a genuine commitment to these principles and an ability to fulfil the obligations arising from membership.

11.2 The application and institutional assessment

An application for membership is submitted to the Council of the European Union.

The European Parliament and national parliaments are notified. The Council acts unanimously after consulting the Commission and obtaining the consent of the European Parliament, which must approve the application by a majority of its component members.

The European Council’s agreed eligibility criteria are also taken into account.

The application is therefore not a purely administrative matter. It involves political assessment by the existing Member States and the institutions of the Union.

The Commission plays a central technical and evaluative role by examining the applicant’s legal system, institutions, economy, and capacity to implement EU law.

11.3 Accession negotiations

Once a state has been recognised as a candidate and the necessary political conditions for negotiations are satisfied, accession negotiations may begin.

The negotiations examine the candidate’s readiness to assume the obligations of membership.

The acquis is divided into policy areas known as negotiating chapters. These cover fields such as:

• the free movement of goods, persons, services, and capital;
• competition policy;
• public procurement;
• agriculture and fisheries;
• transport and energy;
• taxation and customs;
• environmental protection;
• consumer and health protection;
• judicial systems and fundamental rights;
• justice, freedom, and security;
• external relations; and
• financial and institutional provisions.

The candidate must demonstrate not only that its legislation is formally aligned with EU requirements but also that its institutions possess the practical capacity to implement and enforce those requirements.

Negotiations may include transitional arrangements where justified. However, these arrangements must be agreed within the accession framework and cannot be assumed to exist merely because a candidate requests them.

11.4 The accession treaty

Once negotiations are concluded, the terms of accession are recorded in an accession treaty.

The treaty establishes the conditions of admission, transitional arrangements, institutional adjustments, and any specific provisions necessary to integrate the new Member State into the Union.

It must be agreed by the existing Member States and the applicant state. It then requires ratification by all contracting states in accordance with their respective constitutional requirements.

Membership begins on the date specified in the accession treaty, provided that the necessary ratifications have been completed.

This final stage is particularly important. Candidate status, the opening of negotiations, or even the conclusion of negotiations does not itself confer membership. A state becomes a Member State only when the accession treaty enters into force.

12. The Copenhagen Criteria and the Meaning of Membership

12.1 Democracy and the rule of law

The political criteria are among the most important conditions for accession.

A candidate must demonstrate that democratic institutions function effectively, that elections are genuinely competitive, and that constitutional government is respected.

The rule of law requires more than the formal existence of courts and legislation. It involves judicial independence, legal certainty, equality before the law, access to justice, effective remedies, and the ability of public authorities to act within legal limits.

The protection of fundamental rights and minorities is equally important. The Union expects candidate countries to establish effective safeguards against discrimination and to protect individual freedoms through legislation, independent institutions, and functioning judicial systems.

These requirements reflect the constitutional identity of the European Union. Membership presupposes acceptance of a legal and political order based on shared values rather than merely participation in a common market.

12.2 Economic capacity

The economic criteria require a functioning market economy capable of operating within the competitive environment of the internal market.

A candidate must demonstrate that its economic institutions can support competition, investment, and sustainable economic activity. It must also possess sufficient capacity to withstand the pressures generated by participation in the Union’s market.

This does not mean that all Member States must have identical economic structures or income levels. The Union includes economies with different levels of development, industrial specialisations, and public-sector arrangements.

However, accession requires the candidate to demonstrate that its economy can function within a common regulatory and competitive framework.

12.3 The ability to implement the acquis

The legal criterion is particularly demanding.

A candidate must accept the obligations of membership and demonstrate the administrative and institutional capacity to implement EU law.

The acquis covers a vast range of legal subjects. It includes primary law, secondary legislation, international agreements, judicial interpretation, and the principles governing the operation of the Union.

The obligation to implement the acquis is not limited to adopting legislation that resembles EU rules. National authorities must also apply those rules effectively, enforce them consistently, and provide the institutional resources necessary for compliance.

For example, adopting environmental legislation is insufficient if regulatory authorities cannot monitor industrial emissions. Establishing competition rules is insufficient if public authorities cannot investigate unlawful market practices. Transposing judicial cooperation measures is insufficient if courts and prosecutors cannot apply them effectively.

Membership therefore requires a functioning legal and administrative system capable of participating in the Union’s common legal order.

12.4 The Union’s capacity to integrate new members

The Copenhagen framework also recognises the importance of the Union’s own institutional and political capacity.

Enlargement affects the operation of the institutions, the distribution of financial resources, the internal market, and the balance of political interests among Member States.

The Union must therefore consider whether it can integrate a new Member State without undermining the effectiveness of its institutions or the development of its common policies.

This does not mean that the Union can disregard the applicant’s progress or invent arbitrary conditions. It recognises that accession requires agreement between the candidate and the existing Union.

13. The Current Enlargement Process and Candidate Countries

13.1 The continuing enlargement perspective

The European Union’s enlargement policy remains an important part of its external and constitutional development.

Following the accession of Croatia in 2013, no additional state had joined the Union by 1 November 2026. Nevertheless, the enlargement agenda has become increasingly significant in the context of European security, geopolitical competition, democratic stability, and the future relationship between the Union and its neighbouring regions.

The current candidate countries include:

• Albania;
• Bosnia and Herzegovina;
• Georgia;
• Moldova;
• Montenegro;
• North Macedonia;
• Serbia;
• Türkiye; and
• Ukraine.

Their progress is not identical. Some have advanced further in negotiations, while others continue to face substantial political, institutional, or legal obstacles.

Candidate status does not guarantee accession. It establishes a formal relationship with the Union and recognises that the country is being considered within the enlargement framework.

The pace of progress depends on the candidate’s fulfilment of membership conditions, the state of negotiations, and the political decisions required under the Treaties. Consilium+1

13.2 The Western Balkans

The Western Balkans remain a central region in the Union’s enlargement policy.

Albania, Bosnia and Herzegovina, Montenegro, North Macedonia, and Serbia have different institutional histories and levels of progress towards membership.

The region’s European perspective is closely connected with the political and economic transformations that followed the dissolution of Yugoslavia and the conflicts of the 1990s.

The Union’s approach combines accession negotiations with financial assistance, institutional cooperation, economic integration, and support for regional stability.

The rule of law, judicial independence, public administration, corruption, media freedom, and regional cooperation are recurring themes in assessments of the candidate countries.

The enlargement process also has a geopolitical dimension. The Union seeks to strengthen stability in a region geographically surrounded by or closely connected with EU Member States. Nevertheless, geopolitical importance does not remove the requirement that each candidate fulfil the legal and political conditions for membership.

13.3 Ukraine and Moldova

Ukraine and Moldova have acquired particular importance in the enlargement debate following Russia’s full-scale invasion of Ukraine in February 2022.

Both countries applied for EU membership in 2022 and subsequently obtained candidate status.

Their European integration has become closely associated with sovereignty, political independence, institutional reform, and the wider security of the European continent.

The accession process involves difficult questions concerning judicial independence, corruption, public administration, economic reform, and the ability to implement the acquis.

Ukraine also faces the exceptional challenge of pursuing institutional and economic transformation during a war of aggression. The scale of reconstruction and the eventual integration of its economy into the Union would have significant consequences for EU policies and financial arrangements.

The political importance of these applications does not eliminate the legal requirements of accession. The process remains governed by Article 49 TEU and the established membership criteria.

13.4 Türkiye

Türkiye’s relationship with European integration is much older than the current enlargement debate.

It applied for association with the European Economic Community in 1959, and an Association Agreement was signed in 1963. It became a candidate for EU membership in 1999, and accession negotiations formally began in 2005.

However, negotiations have experienced serious political and institutional difficulties. Concerns relating to the rule of law, fundamental rights, judicial independence, and democratic standards have affected the process.

Türkiye also maintains a customs union relationship with the EU in substantial areas of trade in industrial goods, although this does not make it a Member State or confer the full rights and obligations of membership.

Its position demonstrates that a long-standing relationship with the Union, extensive economic cooperation, and candidate status do not automatically result in accession.

14. The Benefits and Responsibilities of EU Membership

14.1 Access to the internal market

One of the most significant advantages of membership is participation in the EU internal market.

The internal market is based on the free movement of goods, persons, services, and capital. It creates a common economic area in which national barriers are progressively removed and common rules govern important aspects of economic activity.

Membership allows businesses to operate within this framework, subject to the relevant EU and national rules. It also gives individuals rights associated with movement, residence, employment, establishment, and the provision of services.

However, internal-market participation also requires compliance with common standards concerning competition, consumer protection, product safety, environmental regulation, and other areas.

The benefits of market access are therefore accompanied by legal and regulatory responsibilities.

14.2 Participation in political and legal decision-making

Member States participate in the Union’s political institutions and contribute to the development of common policies.

They are represented in the Council and European Council, while their citizens elect representatives to the European Parliament.

They also participate in the appointment and composition of the Union’s institutions and in the procedures governing treaty revision, accession, and other major constitutional decisions.

Membership consequently provides more than access to an economic market. It provides a role in shaping the legal and political system that governs that market.

14.3 Financial solidarity and regional development

EU membership also creates access to common financial instruments.

These include cohesion policy funding, agricultural support, research programmes, infrastructure investment, educational initiatives, and other programmes designed to promote economic and social development.

The principle of cohesion is particularly important because economic disparities between regions and Member States remain substantial.

Structural and investment funding seeks to reduce those disparities by supporting infrastructure, employment, innovation, environmental protection, and institutional capacity.

However, financial transfers are governed by eligibility requirements, programme conditions, budgetary rules, and accountability mechanisms. Membership does not create an unconditional right to receive a predetermined amount of funding.

14.4 Legal obligations and financial contributions

Membership also entails substantial responsibilities.

Member States must comply with the Treaties, implement applicable EU legislation, respect the judgments of the Court of Justice, and participate in the financing of the Union’s budget under the agreed system of own resources.

They must cooperate with the Union institutions and avoid measures that undermine the achievement of the Union’s objectives.

Failure to fulfil EU obligations may result in infringement proceedings before the Court of Justice under Articles 258–260 TFEU. In certain circumstances, financial penalties may be imposed.

Membership therefore combines participation in common decision-making with an obligation to respect the legal framework adopted through that system.

15. The Institutional Consequences of Enlargement

15.1 The changing composition of the institutions

Every enlargement has required institutional adaptation.

As the number of Member States increased, the Union had to reconsider the composition of its institutions, the allocation of voting rights, the number of representatives, and the procedures through which common decisions could be adopted.

The original institutional arrangements were designed for six states. They could not remain unchanged as membership expanded to fifteen, twenty-five, twenty-eight, and eventually twenty-seven.

The Treaty of Nice and later the Treaty of Lisbon addressed these challenges by reforming voting procedures, the composition of institutions, and the distribution of responsibilities.

The European Parliament’s membership, for example, is based on degressive proportionality rather than strict population equality. The Council’s qualified majority system combines the number of Member States with their population.

These arrangements attempt to balance the interests of smaller and larger countries while maintaining the capacity to adopt common decisions.

15.2 The effect on decision-making

A larger Union inevitably increases the diversity of national interests.

Member States may differ over economic policy, agriculture, energy, migration, foreign relations, environmental standards, taxation, and the degree of further integration.

The Council must therefore reconcile a greater number of national positions. The European Council must provide political direction across an increasingly diverse membership.

The extension of qualified majority voting has helped the Union avoid requiring unanimity for every legislative decision. Nevertheless, unanimity remains necessary in important areas.

Enlargement has thus strengthened the importance of institutional procedures and the legal rules governing decision-making.

15.3 Diversity and common legal standards

The Union’s enlargement has also increased the diversity of legal traditions and administrative structures.

Civil-law systems predominate in many Member States, while Ireland and Malta have common-law influences and Cyprus has a legal system strongly shaped by common-law traditions. National constitutional arrangements, court structures, administrative procedures, and public-law traditions also differ.

The Union does not require these systems to become identical. Instead, it establishes common legal obligations within areas of Union competence.

This creates a distinctive relationship between national legal diversity and supranational legal uniformity.

Member States retain their constitutional identity and national legal institutions, but they must apply Union law effectively and consistently within the scope of the Treaties.

16. Enlargement as a Political and Constitutional Process

The history of EU membership demonstrates that enlargement has never been purely an economic exercise.

The first enlargement brought the United Kingdom, Ireland, and Denmark into a Community that had developed primarily around the interests of six continental states.

The Southern enlargements helped consolidate democratic government in Greece, Spain, and Portugal.

The 1995 enlargement strengthened the Union’s Northern European dimension.

The 2004 enlargement transformed the political geography of the Union and contributed to the reunification of a continent divided by the Cold War.

The accessions of Bulgaria and Romania extended this process, while Croatia’s accession demonstrated that the enlargement perspective remained open to the Western Balkans.

Brexit, in turn, showed that membership could end through a legally established withdrawal procedure.

These developments reveal three important characteristics of the Union.

First, membership is conditional. A European state must meet legal, political, economic, and institutional requirements.

Second, membership is transformative. Accession changes not only the applicant state but also the institutions, policies, and political balance of the Union.

Third, membership is constitutional. The admission of a new state requires the application of treaty provisions, institutional decisions, legal obligations, and national ratification procedures.

Enlargement is therefore one of the principal mechanisms through which the European Union has developed its identity as a supranational legal order.

17. Key Takeaways

• The European Communities began with six founding states: Belgium, France, the Federal Republic of Germany, Italy, Luxembourg, and the Netherlands.
• The EEC and Euratom Treaties entered into force on 1 January 1958.
• The first enlargement took place in 1973, when Denmark, Ireland, and the United Kingdom joined.
• Greece joined in 1981, following the restoration of democracy.
• Spain and Portugal joined in 1986, strengthening the democratic transformation of Southern Europe.
• Austria, Finland, and Sweden joined in 1995, increasing membership to fifteen.
• The largest enlargement took place on 1 May 2004, when ten states joined, including eight from Central and Eastern Europe.
• Bulgaria and Romania joined on 1 January 2007, bringing membership to twenty-seven.
• Croatia joined on 1 July 2013, becoming the twenty-eighth Member State at the time.
• The United Kingdom withdrew on 31 January 2020 under Article 50 TEU, reducing membership to twenty-seven.
• The Copenhagen criteria establish the principal political, economic, and legal conditions for accession.
• Article 49 TEU is the legal basis for admission of new Member States.
• Accession requires negotiations, agreement on an accession treaty, and ratification by all contracting states.
• Candidate status does not automatically guarantee membership.
• EU membership involves rights and opportunities, but also legal, institutional, financial, and political obligations.
• The Union’s enlargement has repeatedly required institutional reform and adaptation.

18. Frequently Asked Questions

1. How many countries founded the European Communities?

Six countries founded the European Communities: Belgium, France, the Federal Republic of Germany, Italy, Luxembourg, and the Netherlands. They established the ECSC in 1951 and signed the Treaties of Rome in 1957.

2. How did the European Union grow from six to twenty-seven members?

The Union expanded through seven enlargement rounds. Denmark, Ireland, and the United Kingdom joined in 1973; Greece in 1981; Spain and Portugal in 1986; Austria, Finland, and Sweden in 1995; ten states in 2004; Bulgaria and Romania in 2007; and Croatia in 2013. The United Kingdom’s withdrawal in 2020 reduced the membership from twenty-eight to twenty-seven.

3. Which country was the first to join after the six founding states?

Denmark, Ireland, and the United Kingdom joined together on 1 January 1973. They constituted the first enlargement.

4. Why was the 2004 enlargement so important?

It was the largest enlargement in the Union’s history and brought ten countries into the Union, including eight from Central and Eastern Europe. It represented a major step towards overcoming the political division of Europe created during the Cold War.

5. Why did Bulgaria and Romania join in 2007 rather than 2004?

Both countries required additional time to complete institutional, economic, and legal reforms. Their accession was concluded under a separate treaty that allowed them to join on 1 January 2007.

6. When did Croatia join the European Union?

Croatia joined on 1 July 2013. It was the first state from the former Yugoslavia to join the Union and remains the most recent country to have acceded.

7. Why did the United Kingdom leave the European Union?

Following the referendum of 23 June 2016, a majority of voters supported leaving the Union. The United Kingdom subsequently activated Article 50 TEU and formally withdrew on 31 January 2020.

8. What are the Copenhagen criteria?

They are the membership conditions established by the European Council in 1993. They concern stable democratic institutions, the rule of law and human rights, a functioning market economy, and the ability to assume the obligations of EU membership.

9. What is the legal basis for joining the EU?

Article 49 TEU governs accession. It allows a European state that respects the values in Article 2 TEU and is committed to promoting them to apply for membership.

10. Does candidate status guarantee EU membership?

No. Candidate status is an important stage in the accession process, but membership requires the fulfilment of the accession conditions, the conclusion of negotiations, agreement on an accession treaty, and ratification by all contracting states.

11. Are all EU Member States part of the euro area?

No. Membership of the European Union and membership of the euro area are distinct. Some Member States have not yet adopted the euro, while Denmark has a specific treaty-based opt-out.

12. Are all EU Member States part of the Schengen area?

No. The Schengen area is a distinct legal arrangement. It includes most EU Member States as well as certain non-EU countries associated with the system.

13. Can a country leave the European Union after joining?

Yes. Article 50 TEU establishes a formal withdrawal procedure. The United Kingdom’s departure in 2020 was the first practical application of this provision.

14. Which countries are currently candidates for EU membership?

As of 1 November 2026, the candidate countries include Albania, Bosnia and Herzegovina, Georgia, Moldova, Montenegro, North Macedonia, Serbia, Türkiye, and Ukraine. Their progress towards accession differs considerably.

15. Why does enlargement require institutional reform?

An increase in membership affects voting arrangements, representation, the composition of institutions, budgetary priorities, and the ability to adopt common legislation. Successive enlargements have therefore required adjustments to the Union’s institutional and constitutional framework.

Conclusion

The development of the European Union from six founding states to twenty-seven Member States reflects the transformation of European integration from a limited post-war economic project into a major supranational political and legal order.

Each enlargement has taken place within a distinct historical context. The first brought the Communities beyond their original continental core. The accessions of Greece, Spain, and Portugal connected European integration with democratic consolidation in Southern Europe. The 1995 enlargement strengthened Northern European participation, while the 2004 enlargement represented a historic step towards overcoming the division of the continent. Bulgaria and Romania extended this transformation, and Croatia’s accession demonstrated the continuing relevance of the enlargement process in the Western Balkans.

Brexit introduced a different constitutional experience: the withdrawal of a Member State from a system that had become deeply integrated through decades of common legislation, institutional cooperation, and economic interdependence.

The legal framework governing membership is designed to balance openness with conditionality. Article 49 TEU provides the formal basis for accession, while the Copenhagen criteria establish the principal standards that candidates must satisfy. These requirements reflect the understanding that membership is not simply a matter of geography or political preference. It involves acceptance of a common legal order, the protection of democratic values, the capacity to implement European legislation, and participation in a system of shared institutions.

Enlargement has also transformed the Union itself. A larger membership has increased its economic and political influence, but it has also created greater institutional complexity and a wider range of national interests. The Union must continuously reconcile diversity with common legal standards, national constitutional autonomy with supranational authority, and the principle of equal membership with the practical requirements of collective decision-making.

The history of enlargement therefore reveals an essential feature of the European Union: it is an evolving legal and political community whose boundaries, institutions, and responsibilities have developed together. The admission of new states is not an external addition to European integration. It is one of the principal processes through which the Union has acquired its present form.

Official and Further Sources

1. European Commission — History of Enlargement: From 6 to 27 Members. Official historical overview of the successive enlargement rounds and the development of the Union’s membership. Enlargement and Eastern Neighbourhood

2. Council of the European Union — Timeline of Accession of EU Member States. Official chronology of the accession dates of the Member States and the United Kingdom’s withdrawal. Consilium

3. EUR-Lex — EU Enlargement Policy. Overview of the legal basis and historical development of the Union’s enlargement policy. EUR-Lex

4. European Union — EU Enlargement. Explanation of the membership criteria and the principal stages of accession. European Union

5. European Commission — Enlargement Countries. Official information about the accession process, candidate countries, and the Copenhagen criteria. European Commission

6. Council of the European Union — EU Enlargement. Information about candidate countries, accession negotiations, and the role of the Council in enlargement. Consilium+1

7. European Parliament — The Enlargement of the Union. Historical and institutional analysis of the enlargement process. European Parliament

8. EUR-Lex — Treaty on European Union, Article 49. The treaty provision governing applications for EU membership.

9. EUR-Lex — Member States. Official overview of the current Member States and the legal consequences of membership. EUR-Lex

10. European Union — EU Countries. Official country profiles, including accession dates and participation in the euro and Schengen areas. European Union