The European Union's enlargement that took place on 1 May 2004 is in many respects unprecedented. First and foremost, its sheer scale outnumbers previous enlargements – twelve countries were in the process of accession negotiations, and ten countries have joined: Poland, Hungary, the Czech Republic and Slovakia from the so-called Vishegrad block; Estonia, Latvia and Lithuania from the Baltic region; Slovenia from the former Yugoslavia; and Malta and Cyprus from the Mediterranean. In previous rounds of enlargement, up to three countries have joined at a time: the United Kingdom, Ireland and Denmark in 1973; Greece in 1980; Portugal and Spain in 1986; and Austria, Sweden and Finland in 1995. Further, this enlargement has immense political significance as ‘a reunification of Europe’: the enlargement project aims to rectify historical injustice for countries that had suffered under the Soviet yoke, and bolster the zone of political stability and security in Europe. Unlike past enlargement practice, a pre-accession process of ‘unprecedented length and complexity’ was designed, involving a sophisticated set of pre-accession instruments, strategies and policies.

For the CEE accession countries, membership of the European Union, along with joining NATO, has formed the main foreign policy goal since the breakdown of the Communist regime. Although in the early years EU membership was not a self-evident path, and alternative relationships, such as forming an economic area or a loose form of confederation, were offered by the EU's leaders, the CEE countries insisted on the prospect of full membership in order to avoid remaining in a geopolitical ‘grey zone’.