The European Commission’s Agenda 2000 package laid out how the EU would prepare its budget and policies for taking in new members from Central and Eastern Europe. It linked enlargement to reforms in major spending areas like agriculture and regional development, and to stronger pre-accession support. This created a practical plan to move from political promises to a managed enlargement process.
At Luxembourg, EU leaders formally launched an accession process for the ten Central and Eastern European applicants and Cyprus. The Council also set out how the process would work and confirmed that all applicants were “destined to join” if they met the same criteria. This decision turned enlargement into an organized, step-by-step negotiation project.
The EU opened accession negotiations with Cyprus and five Central/Eastern European countries: the Czech Republic, Estonia, Hungary, Poland, and Slovenia. Negotiations focused on adopting the EU’s acquis—the full body of EU laws and rules that members must follow. This marked the start of detailed, chapter-by-chapter work needed for membership.
The European Commission published “regular reports” assessing how each applicant country was progressing toward membership. These reports became a key accountability tool, documenting reforms and gaps in areas like the economy, public administration, and the rule of law. They helped shape what each country needed to fix before closing negotiations.
EU leaders in Berlin reached an overall agreement on Agenda 2000, including the financial framework meant to support enlargement. This helped ensure the EU budget could handle new members and provided clearer funding rules for pre-accession assistance and future EU spending. The agreement reduced uncertainty about whether enlargement was financially workable.
At Helsinki, the EU decided to extend the accession negotiations process to six more applicants: Bulgaria, Latvia, Lithuania, Malta, Romania, and Slovakia. This reduced the risk of a permanent “first-class vs. second-class” track among applicants and widened the enlargement project. It also confirmed that enlargement would cover much of Central and Eastern Europe, not just a small subset.
The EU opened negotiations with Bulgaria, Latvia, Lithuania, Malta, Romania, and Slovakia. This brought most candidates into the same formal negotiation system, using standardized chapters of the EU legal framework. The move helped align reforms across countries and set the stage for a large “big bang” accession rather than many small ones.
EU member states signed the Treaty of Nice to reform institutions for a larger union. The treaty adjusted decision-making rules, including expanding qualified-majority voting in some areas and changing institutional arrangements so the EU could function with many more members. These institutional changes were widely treated as necessary groundwork for the 2004 enlargement.
EU leaders adopted the Laeken Declaration, acknowledging that enlargement would increase pressure on EU decision-making and public trust. It created the “Convention on the Future of Europe” to debate and draft proposals for institutional reform. While not an accession step by itself, it was part of the political effort to keep an enlarged EU workable and legitimate.
EU leaders concluded accession negotiations with ten countries: Cyprus, the Czech Republic, Estonia, Hungary, Latvia, Lithuania, Malta, Poland, Slovakia, and Slovenia. They also fixed 1 May 2004 as the accession date. This decision effectively finalized the political deal and moved the process into treaty drafting and ratification.
After ratification in all existing member states, the Treaty of Nice entered into force. This mattered for enlargement because it updated EU institutional rules meant to support decision-making in a much larger union. With Nice active, the EU had a stronger legal and institutional base for admitting ten new members.
The EU and the ten acceding states signed the Treaty of Accession in Athens, turning the Copenhagen agreement into a binding legal text. The treaty set out membership terms and transitional arrangements (temporary rules) for areas such as labor movement and specific sector policies. Signature was a major shift from negotiation to final ratification steps in each country.
Poland held a two-day referendum on EU accession, and voters approved membership by a large margin. Because Poland was the largest of the Central and Eastern European candidates, its vote was a major political signal that enlargement had broad public backing in a key acceding state. Successful referendums and parliamentary approvals across candidates were essential for the treaty to enter into force.
On this date, the EU expanded from 15 to 25 members as ten states joined: Cyprus, the Czech Republic, Estonia, Hungary, Latvia, Lithuania, Malta, Poland, Slovakia, and Slovenia. The enlargement completed a major phase of post–Cold War European integration by bringing many former communist states into the EU’s single market and institutions. It also shifted EU economic policy and regional development priorities toward a larger, more diverse membership.
2004 'Big Bang' Enlargement: Central and Eastern Europe (1997-2004)