On 1 May 2004, the leaders of 10 acceding countries presented their flags to Pat Cox, the then European Parliament president.
The EU grew from 15 to 25 with the inclusion of Cyprus, Czech Republic, Estonia, Hungary, Latvia, Lithuania, Malta, Poland, Slovakia, and Slovenia.
The bloc now includes 27 countries - Croatia was the most recent addition, joining in 2022. The UK chose to leave the group in June 2016.
The 10 member states represented a 20 percent increase in population, and the EU territory increased by almost the same percentage.
The total GDP grew by about nine percent, while the GDP per capita decreased.
Michael Emerson, Associate Senior Research Fellow at CEPS Brussels, said the expansion had economic and societal benefits, but less so in the political field.
''Economics have gone very well. All of the new member states have been growing faster than all of the old member states. Financial stability has been reasonably good,'' he said. ''On the people's side, the migration movements in and out have been happening very freely and in an orderly manner. Now the politics - there are a few bumps on the road there, otherwise known as Poland and Hungary and possibly Slovakia.''
In 2017, the European Commission initiated a procedure under Article 7 in response to the risks to rule of law and EU values in Poland. The European Parliament backed this move in a resolution in March 2018.
Parliament triggered the Article 7 procedure for Hungary in September 2018.
Earlier this year, Adam Bodnar, Polish Public Prosecutor General, presented an 'action plan' at a meeting of European affairs ministers in Brussels, consisting of nine bills aimed at restoring judicial independence.
The overture is part of the diplomatic reset that Prime Minister Donald Tusk has spearheaded since taking office in December.
Poland has been under Article 7 since 2017 due to systematic breaches of fundamental values and the continued erosion of judicial independence.
Hungary has been under the first phase of Article 7 since 2018 over the democratic backsliding overseen by Prime Minister Viktor Orban, who is accused of weakening judicial independence, perpetuating cronyism, diluting media pluralism, abusing emergency powers, passing anti-LGBT legislation and hindering asylum rights.
Meanwhile, nine more countries are vying for membership - Albania, Bosnia and Herzegovina, Georgia, Moldova, Montenegro, North Macedonia, Serbia, Turkiye, and Ukraine.
To be accepted into the group, each contender must work to make the values and laws of the EU their own.
Last year, the six Western Balkans countries were presented with a growth plan, and offered access to parts of the EU single market in exchange for substantial reforms, as a stepping stone to full EU membership.
On Monday, European Council President Charles Michel said the EU must get bigger or risk facing a "new Iron Curtain" along its eastern flank.
The remark comes as Russia's war with member-in-waiting Ukraine intensifies.
"It would be extremely dangerous if you would have an unstable neighbourhood with a lack of prosperity or lack of economic development. These are our common interests - of candidate countries and the EU - to make progress, to speed up," he said.
Michael Emerson said there is a possibility those new countries will violate the bloc's values and interests.
''For the Balkans, it can proceed with safeguard mechanisms, I would say, on the political side. Of course, Ukraine is a unique case, a big, big one, and we don't know how the war is going to end,'' he said.
Earlier this month, exclusive IPSOS/Euronews poll found 45 percent of voters across the EU are in favour of Ukraine joining the bloc, while 35 percent are openly against it, and 20 percent are undecided.
The member state most opposed to Ukraine's accession is Hungary where 54 percent of respondents are against it and 18 percent are in favour.
The war-torn country and neighbouring Moldova put in their bid to become EU members within weeks of Russia's invasion of Ukraine and acquired candidate status in record time.
The EU agreed to start accession negotiations with Ukraine in late 2023.
The country is required to strengthen the fight against corruption, adopt a comprehensive law on lobbying, and finalise the reform of the legal framework for national minorities.
