Germany, France, Italy, Belgium, the Netherlands and Luxembourg created the predecessor to the EU on March 25, 1957 with the signing of the Treaty of Rome. The European Economic Community (EEC) aimed to promote integration for internal and external trade. The EEC and two sister organizations governing coal, steel and nuclear cooperation were collectively known as the European Communities (EC).

The United Kingdom was initially reluctant to join the new European club but changed its mind in the 1960s. Its first two accession attempts failed in the face of strong French opposition. Paris eventually relented, and the UK became an EC member along with Ireland and Denmark in 1973. A majority of British voters approved the decision to join the bloc in a 1975 referendum.

Greece became the EC's tenth member in 1981 after six years of negotiations. The Mediterranean country's transition to democracy following the collapse of a military dictatorship in 1974 paved the way for its application to the bloc. Greece's admission was controversial. The country was relatively poor and some EC members worried about economic competitiveness and an influx of migrants.

Spain and Portugal followed Greece five years later. They were, like Greece, new democracies. Spain transitioned following the death of former dictator Francisco Franco in 1975. Free elections were also held in Portugal in that year after the fall of the former authoritarian government. Portuguese Prime Minister Antonio Costa (pictured) marked the 30th anniversary of the accession in 2016.

The EC became the modern-day European Union after its members signed the Treaty of Maastricht in 1992. The first countries to join the new bloc were Austria, Sweden and Finland in 1995. All three countries had been officially neutral during the Cold War and were not members of the North Atlantic Treaty Organization (NATO) alliance.

The EU saw its biggest expansion with the addition of ten countries — Czech Republic, Estonia, Cyprus, Latvia, Lithuania, Hungary, Malta, Poland, Slovakia and Slovenia — on May 1, 2004. It was an important landmark in those countries' economic and political development. All were newcomers to democracy and capitalism following the collapse of communism less then two decades before.

Romania and Bulgaria planned to join the EU in 2004. But the bloc delayed accession amid their slow progress on completing judicial and political reforms. Both countries continue to be among the bloc's poorest and most corrupt members. GDP per capita in Romania was 63 percent of the EU average in 2017. In Bulgaria, GDP per capita was only 49 percent of the EU average.

The most recent round of EU enlargement saw Croatia join in 2013. The small country on the Adriatic Coast followed Slovenia as the second country to emerge from the bloody breakup of Yugoslavia in the 1990s and join the EU. Montenegro and Serbia, which were also part of Yugoslavia, have been negotiating future EU membership since 2012 and 2014.

The EU decided in late June 2018 to open EU membership negotiations with an additional two Balkan countries, the Former Yugoslav Republic of Macedonia (FYRM) and Albania. Brussels has not yet decided to open negotiations with the remaining former Yugoslav countries, Bosnia-Herzegovina and Kosovo, which the bloc considers "potential candidate countries" for future EU membership.