69 Years of the EU: Every Country That Joined, and the One That Left

The European Union has grown from six founding members to twenty-seven in sixty-nine years — and lost exactly one along the way. The six founders (Belgium, France, West Germany, Italy, Luxembourg, the Netherlands) signed the Treaty of Rome in 1957, creating the European Economic Community. Seven further waves of accession followed, the largest by far in 2004, when ten countries joined at once.

The only country to ever leave

The United Kingdom joined in the first enlargement, in 1973, alongside Ireland and Denmark. It left on 31 January 2020 — the only member state in the EU’s history to withdraw. No accession has undone that: the total sits at 27, not 28, and has for six years.

Nine countries currently hold official candidate status. Montenegro is furthest along in the accession process, with several negotiating chapters provisionally closed as of mid-2026, but no accession date has been set for any candidate.

Reading the sequence

The membership count didn’t climb steadily — it moved in eight distinct jumps: 6, 9, 10, 12, 15, 25, 27, 28, and back to 27 after the UK left. Each wave after the six founders added a different number of countries: three in 1973 (the UK, Ireland and Denmark), one in 1981, two in 1986, three in 1995, ten in 2004, two in 2007, and one more in 2013 — the peak of 28 members, before the UK’s 2020 withdrawal brought the total back down to 27. The 2004 wave alone added two-thirds again to the club’s size in a single year — 15 members growing to 25 — ten countries admitted together against a background of one, two or three at a time in every other wave before or since. That's also why the nine current candidate countries don’t map onto a fixed timetable: every prior wave, including 2004’s, closed on its own schedule, and Montenegro’s provisionally-closed chapters mark it as further along than any other candidate, not as a guarantee of when the next member actually joins.