On February 7, 1992 the Maastricht Treaty was signed in Maastricht, Netherlands creating the European Union. The 12 original signatories were Belgium, Denmark, France, Germany, Greece, Ireland, Italy, Luxembourg, Netherlands, Portugal, Spain, and the United Kingdom.
Finland and Sweden joined in 1995 and Austria in 1999.
In 2004, the EU saw its biggest enlargement when Cyprus, the Czech Republic, Estonia, Hungary, Latvia, Lithuania, Malta, Poland, Slovakia and Slovenia joined.
Bulgaria and Romania joined in 2007, followed by Croatia in 2013.
Conspicuously absent is Norway, which has twice rejected membership in public referendums.
In March, 2017 the UK notified the European Council of its intention to leave the EU by triggering Article 50 of the Lisbon Treaty. The Treaty provides two years-730 days-for a country to extricate itself. Colloquially known as Brexit, complicated negotiations are underway.
Read more about it here.

The European Union flag was designed by Arsène Heitz and the 12 stars stand for the 12 original signatories. Hmmmm. . .will it have to be redesigned?