2004 enlargement of the European Union

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The largest enlargement of the European Union (EU), in terms of number of states and population, took place on 1 May 2004.

The simultaneous accessions concerned the following countries (sometimes referred to as the "A10" countries^[1][2]): Cyprus, the Czech Republic, Estonia, Hungary, Latvia, Lithuania, Malta, Poland, Slovakia, and Slovenia. Seven of these were part of the former Eastern Bloc (of which three were from the former Soviet Union and four were and still are member states of the Central European alliance Visegrád Group). Slovenia was a non-aligned country prior to independence, and it was one of the former republics of Yugoslavia (together sometimes referred to as the "A8" countries), and the remaining two were Mediterranean island countries, both member states of the Commonwealth of Nations.

Part of the same wave of enlargement was the accession of Bulgaria and Romania in 2007, who were unable to join in 2004, but, according to the European Commission, constitute part of the fifth enlargement.