Irish Prime Minister Bertie Ahern said in a pre-summit press conference that he considers completing agreement on a new constitution the first great challenge facing the enlarged EU. Ireland is the present holder of the EU's rotating presidency.
Although EU governments still disagree on key issues, he said, solutions can be found, in his words, "with the necessary political will."
Ahern said that today's enlargement already demonstrates the determination of the new members.
"To the people of Europe who are joining us today in the European Union, I extend the hand of friendship," Ahern said. "It was your democratic choice and your own efforts that made this day happen. Today marks the triumph of your determination and perseverance over the legacy of history."
Ahern has said he wants the next EU summit, in mid-June, to seal agreement on the treaty.
Eight formerly communist countries -- the Czech Republic, Estonia, Hungary, Latvia, Lithuania, Poland, Slovakia and Slovenia -- along with Cyprus and Malta became EU members today.
At the border where Germany meets Poland and the Czech Republic, the countries' three leaders today made the symbolic first dig in a road that will connect the three states.
Czech Prime Minister Vladimir Spidla said the act is symbolic of Europe's new unity.
"Today we are launching the construction of a road, the construction of a connection, but it's really about much more," Spidla said. "Because it's not just a road, but cooperation in education, cooperation in euro regions, cooperation of fire services. There are thousands of examples of this cooperation, both tangible and intangible."
But even as they marked this historic enlargement, several EU leaders were already looking forward to the next round of expansion.
European Commission President Romano Prodi today said talks with Bulgaria -- which wants to join in 2007 -- are on track.
"Negotiations with Bulgaria are going on well, very well. I don't see any major obstacles that make me think there could be any delay," Prodi said.
Polish President Aleksander Kwasniewski said the bloc should remain open to admitting other former communist countries -- from Ukraine to the Balkans.
"Today, Poland is returning to its European family," Kwasniewski said. "But the door to the European Union must be left open. All those whose ambitions, hopes and dreams lie with the European Union are invited."
The first big challenge to the expanded bloc will be reaching agreement on a new constitution, which is meant to streamline decision-making.
Officials are hoping for a deal at the EU's June summit, but differences remain, particularly over voting rights.
The enlargement marks the final closing of Europe's east-west divide, 15 years after the Berlin Wall fell and 60 years after the end of World War II. It increases the bloc's membership to 25 members, and its population by 75 million people. The EU now has some 450 million people, making it the world's largest free trade area.
(compiled from wire reports)
to see RFE/RL's "EU Expands Eastward" webpage.)