How does a country join the European Union, and why is each enlargement debated?

A country joins the European Union by meeting set conditions, rewriting its laws to match European law, and winning the agreement of every current member state and the European Parliament. The conditions are the Copenhagen criteria, set in June 1993: stable institutions guaranteeing democracy, the rule of law, human rights and protection of minorities, a functioning market economy, and the capacity to cope with competition inside the Union.

The steps go like this. A country typically signs an association agreement first, then applies. The Council asks the Commission for an opinion on its readiness, and in screening the Commission and the candidate compare their laws with the EU's. When negotiations end, a Treaty of Accession is signed and has to be ratified by all member states, the Union's institutions and the candidate. The whole process typically takes about a decade, though Sweden, Finland and Austria took only a few years.

The biggest single enlargement came on 1 May 2004, when eight Central and Eastern European countries plus Malta and Cyprus joined. It was the largest by people and number of countries but not by GDP, and some older members were concerned about how much less developed the new members were.

Enlargement has always had its critics. French president Charles de Gaulle opposed British membership, and François Mitterrand opposed Greek, Spanish and Portuguese membership, fearing the former dictatorships were not ready. After the 2004 wave, public opinion in Europe turned against further expansion, and former Commission President Romano Prodi favoured offering neighbours "everything but institutions": deep cooperation without adding strain on the EU's institutions.