Millions of Bulgarians and Romanians awoke today as citizens of the European Union after a night of fireworks and street parties celebrating their countries' entry into the bloc at midnight.
Deemed too politically and economically backward for membership during the EU's first eastward expansion in 2004, the Black Sea neighbours squeezed through the door in what political analysts say was the last chance to join this decade.
The accession of the poor, ex-communist duo raises the EU's membership to 27 states, almost half of them former eastern bloc countries cut off from the West by the Iron Curtain until 1989.
"Today a dream came true, a dream of generations of Bulgarians who have always wanted to live together with the free European peoples in peace and prosperity," Bulgarian Prime Minister Sergei Stanishev said at an outdoor concert in Sofia.
The EU's new borders will stretch from the Atlantic and Baltic in the west and north to the Black Sea in the southeast.
Romania - the larger of the two - and Bulgaria will together boost the EU's population by 30 million, to 490 million, but will add just 1 per cent to its economic output.
Their membership has sparked debate over the EU's eventual borders, with some member states fearing further expansion could bring waves of immigration and crime that could drive their citizens out of jobs and lead to instability.
Other EU hopefuls such as Turkey and countries in the western Balkans now face the prospect of a long wait.
Taoiseach Bertie Ahern said today's accession was a "cause for celebration".
"I am very pleased, on behalf of the Government and the Irish people, to welcome [Bulgaria and Romania] as equal partners within our Union," he said. "Enlargement has been one of the Union's most successful policies. It has created a region of peace, stability and prosperity across the continent of Europe.
"I have every confidence that accession to the Union will be as significant and as positive for Bulgaria and Romania as it has been for Ireland."
Minister for Foreign Affairs Dermot Ahern said today's move was "another important step in bringing together the peoples of Europe".