Snapshot

The European Union developed through successive treaties and enlargements from the post-war project announced in the Schuman Declaration of 9 May 1950. Pooling coal and steel led to a community of six states; later treaties created a customs union, an internal market, common policies, EU citizenship and an economic and monetary union. The Union has had 27 member states since the United Kingdom withdrew on 31 January 2020.

Legal Anchors

• •TEU Preamble and Article 1 — establishment of the Union and conferral of competences by the member states.
• •TEU Article 49 — accession clause used by every joining state since 1973.
• •TEU Article 50 — withdrawal clause used by the United Kingdom in 2017.
• •Full consolidated TEU (OJ C 202, 7.6.2016) and TFEU.

Why It Matters

Three features of present-day EU design are clearer in their historical context: the Commission's central role reflects the supranational model of the ECSC High Authority; unanimity in much of foreign policy preserves strong member-state control; and successive enlargements have repeatedly raised the question of how widening membership affects deeper integration.

Core Concepts — Reorganised by Theme

1. The peace-through-interdependence idea. The approach associated with Jean Monnet pursued integration through concrete, limited projects that could create practical solidarity and support further cooperation. Monnet and his collaborators prepared the proposal that French Foreign Minister Robert Schuman presented on 9 May 1950. In a separate landmark speech at Zurich in 1946, Winston Churchill had called for a form of "United States of Europe"; the United Kingdom did not join the first European Communities.

2. The institutional skeleton appears in 1951. The Treaty of Paris (signed 18 April 1951, in force 23 July 1952, expired 23 July 2002) created the European Coal and Steel Community among the Six — France, West Germany, Italy, Belgium, the Netherlands and Luxembourg. Its four organs prefigure today's EU: a High Authority (predecessor of the Commission), a Common Assembly (predecessor of the Parliament), a Special Council of Ministers (predecessor of the Council of the EU) and a Court of Justice. Coal and steel were the raw materials of war; placing them under common control made secret rearmament impractical — a structural rather than rhetorical guarantee of peace.

3. The common market is launched in Rome (1957). Two treaties were signed on 25 March 1957 and entered into force on 1 January 1958: one created the European Economic Community (EEC), with a customs union and progressive freedom of movement for goods, persons, services and capital; the other created Euratom for civil nuclear cooperation. Euratom remains in force today as a separate legal community using the same institutions as the EU.

4. Growing pains in the 1960s and 1970s. The Common Agricultural Policy began in 1962 as one of the EEC's first common policies. During the Empty Chair Crisis, France stayed away from Council meetings from July 1965 until January 1966 amid disagreement over financing and majority voting. Under the Luxembourg Compromise of January 1966, the six governments agreed to keep seeking mutually acceptable solutions when very important interests were invoked, while recording continuing disagreement over what should happen if no agreement was reached. The Merger Treaty (in force from 1967) gave the three Communities (ECSC, EEC and Euratom) a single Commission and Council. The customs union was completed on 1 July 1968, eighteen months ahead of schedule. The United Kingdom, Ireland and Denmark joined in 1973, the first enlargement.

5. The Single Market shift (1985–1992). Three milestones converged. The Schengen Agreement (1985) was signed outside the Community framework by five members and began the long process of dismantling internal border checks. The Single European Act (signed 1986, in force 1 July 1987) set a hard deadline of 31 December 1992 for completing the internal market, introduced qualified majority voting for market legislation, and re-energised what had become a stagnant project. Direct elections to the European Parliament had begun a few years earlier (1979). The fall of the Berlin Wall (1989) changed the strategic horizon: a reunified Germany was about to anchor a wider Union.

6. Maastricht — the Union itself appears (1992). The Treaty signed on 7 February 1992 and in force from 1 November 1993 established the European Union. It introduced EU citizenship (automatic for nationals of member states), set the path to a single currency (the euro), created a Common Foreign and Security Policy as a separate pillar, and codified subsidiarity as a principle limiting EU action. The separate Copenhagen European Council of June 1993 then set the political, economic and acquis-related criteria for future enlargement; the Madrid European Council added the administrative-capacity requirement in 1995. The three-pillar structure survived until Lisbon.

7. Iterations: Amsterdam (1999), Nice (2003), the failed Constitution (2005), Lisbon (2009). Amsterdam brought Schengen into the EU framework, strengthened the AFSJ and created the office of High Representative for the CFSP. Nice prepared institutional capacity for big-bang enlargement. The Treaty establishing a Constitution for Europe, signed in 2004, collapsed after French and Dutch voters rejected it in referenda (2005). Its substance — minus the constitutional vocabulary — was repackaged as the Treaty of Lisbon, signed 13 December 2007 and in force from 1 December 2009.

8. What Lisbon changed. It abolished the pillar structure and gave the EU legal personality; created the full-time office of President of the European Council (first held by Herman Van Rompuy); combined external-relations responsibilities in the High Representative / Vice-President of the Commission role (first held under Lisbon by Catherine Ashton); made the Charter of Fundamental Rights legally binding with the same legal value as the treaties; introduced Article 50 TEU on withdrawal and the European Citizens' Initiative; and gave national parliaments a formal role in monitoring subsidiarity.

Numbers & Dates

Recent Developments (2024–2026)

Bulgaria adopted the euro on 1 January 2026, becoming the twenty-first member of the euro area. Internal land-border controls with Bulgaria and Romania were lifted on 1 January 2025, completing their entry into the Schengen area. Ukraine and Moldova formally opened accession negotiations on 25 June 2024; each opened negotiations on Cluster 1, Fundamentals, at accession conferences on 15 June 2026.

Deep Dive — Roads Not Taken and the People Who Built Europe

Integration advanced through failures as well as treaties; several formative episodes are less well known than the treaty milestones but shaped the Union's design.

• •The Fouchet Plans (1961–1962). France under de Gaulle proposed a "Union of States" — an intergovernmental political union run by unanimity, outside the Community method. The Netherlands and Belgium blocked it, fearing it would sideline the supranational institutions and exclude the UK. Its failure preserved the Community method and left political cooperation to develop later through European Political Cooperation (1970) and, eventually, the CFSP.
• •The Merger Treaty (signed 1965, in force 1967). Fused the separate executives of the ECSC, EEC and Euratom into a single Commission and a single Council — the institutional shape the Union still has.
• •The Fontainebleau European Council (June 1984). Settled the British budgetary question ("I want my money back") by creating the UK rebate, and set up the Adonnino Committee (People's Europe — symbols, Erasmus groundwork) and the Dooge Committee (institutional reform). Fontainebleau unblocked the relaunch that produced the Single European Act.
• •The Spinelli draft Treaty (1984). The European Parliament, inspired by Altiero Spinelli (co-author of the 1941 Ventotene Manifesto), adopted a draft Treaty establishing the European Union. Never ratified, it shaped the agenda that led to Maastricht.

Founding and formative figures to know: Jean Monnet (Planning Commissioner, first President of the ECSC High Authority), Robert Schuman (9 May 1950 declaration), Konrad Adenauer, Alcide De Gasperi and Paul-Henri Spaak (the Spaak Report prepared the Treaties of Rome), Altiero Spinelli (federalist), Simone Veil (first President of the directly elected European Parliament, 1979) and Jacques Delors (Commission President 1985–1995, single market and EMU blueprint). The EU's "founding fathers" gallery also honours Winston Churchill, Johan Willem Beyen and Sicco Mansholt.

Common Confusions

• •Council of Europe ≠ EU. The Council of Europe (founded 5 May 1949, seat in Strasbourg) is an entirely separate organisation with 46 member states. It runs the European Convention on Human Rights and the European Court of Human Rights. It is not an EU institution.
• •"Treaty of Rome" usually refers to the EEC treaty, but technically there were two Rome treaties in 1957 (EEC and Euratom). After the Lisbon renumbering, what was the EEC Treaty / Treaty establishing the European Community is now the TFEU.
• •Europe Day is 9 May (the Schuman Declaration anniversary), not 1 May.
• •The "Six" did not include the United Kingdom. The UK declined to join the ECSC in 1951 and the EEC in 1957; it only acceded in 1973.

Capsule Glossary

• •Schuman Declaration — proposal of 9 May 1950 to pool Franco-German coal and steel under a supranational High Authority. Treated as the EU's founding act.
• •Monnet method / engrenage — incrementalist integration: small concrete steps generate logical pressure for further steps.
• •Acquis communautaire — the cumulative body of EU rights and obligations (treaties, secondary law, case law, agreements) that a candidate state must adopt before joining.
• •Subsidiarity — Article 5 TEU principle governing Union action in areas that do not fall within exclusive EU competence.
• •Copenhagen Criteria — political, economic and acquis conditions a candidate must meet (1993).
• •Luxembourg Compromise — 1966 political arrangement to continue seeking mutually acceptable solutions when very important national interests were invoked; the governments recorded different views on how long discussions should continue.
• •Article 50 TEU — the exit clause introduced by Lisbon, first triggered by the UK in 2017.

Cross-References

• •Treaty hierarchy and supremacy → Chapter 2
• •The Charter and the values listed in Article 2 TEU → Chapter 3
• •Article 7 procedure (rule-of-law sanctions) → Chapters 3 and 17
• •The 2024 fiscal-rules reform → Chapter 13

Primary Sources

• •TEU (consolidated), OJ C 202, 7.6.2016 — eur-lex.europa.eu
• •Schuman Declaration — european-union.europa.eu/principles-countries-history/history-eu/1945-59/schuman-declaration-may-1950_en
• •Bulgaria's euro-area accession — consilium.europa.eu
• •Bulgaria and Romania: lifting of internal land-border controls — consilium.europa.eu
• •Accession conferences with Ukraine and Moldova, 15 June 2026 — Ukraine and Moldova

---