For more than six decades, the European Union has expanded to encompass 28 member countries, and Britain's departure marked its first contraction.

Here is a timeline:

-- 1957: Six countries -- Belgium, France, Germany, Italy, Luxembourg and the Netherlands -- sign the Treaty of Rome establishing the European Economic Community (EEC), the precursor to the EU.

-- 1973: Britain, Denmark and Ireland join the bloc, taking the number of members to nine.

-- 1981: Greece becomes the 10th member.

-- 1986: Portugal and Spain join.

-- 1995: Austria, Finland and Sweden enter what has evolved into the EU, which now has 15 members. Norway rejects accession in a referendum.

-- 2004: Eight ex-communist Eastern European countries and two Mediterranean nations join: Cyprus, the Czech Republic, Estonia, Hungary, Latvia, Lithuania, Malta, Poland, Slovakia and Slovenia. The bloc swells to 25 members.

-- 2007: Bulgaria and Romania take the EU to 27 member nations.

-- 2013: Croatia becomes the 28th.

-- 2020: Britain becomes the first member state to leave the EU, after a seismic 2016 referendum vote to quit the bloc. It finally leaves the bloc's single market and customs union on the last day of 2020.

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