Preliminary statement:
The evolution of what is today the European Union (EU) from a regional economic agreement among six neighboring states in 1951 to today's hybrid intergovernmental and supranational organization of 28 countries across the European continent stands as an unprecedented phenomenon in the annals of history. Dynastic unions for territorial consolidation were long the norm in Europe; on a few occasions even country-level unions were arranged - the Polish-Lithuanian Commonwealth and the Austro-Hungarian Empire were examples. But for such a large number of nation-states to cede some of their sovereignty to an overarching entity is unique. Although the EU is not a federation in the strict sense, it is far more than a free-trade association such as ASEAN, NAFTA, or Mercosur, and it has certain attributes associated with independent nations: its own flag, currency (for some members), and law-making abilities, as well as diplomatic representation and a common foreign and security policy in its dealings with external partners. Thus, inclusion of basic intelligence on the EU has been deemed appropriate as a separate entity in The World Factbook. However, because of the EU's special status, this description is placed after the regular country entries.

Background:
Following the two devastating World Wars in the first half of the 20th century, a number of far-sighted European leaders in the late 1940s sought a response to the overwhelming desire for peace and reconciliation on the continent. In 1950, the French Foreign Minister Robert SCHUMAN proposed pooling the production of coal and steel in Western Europe and setting up an organization for that purpose that would bring France and the Federal Republic of Germany together and would be open to other countries as well. The following year, the European Coal and Steel Community (ECSC) was set up when six members - Belgium, France, West Germany, Italy, Luxembourg, and the Netherlands - signed the Treaty of Paris. The ECSC was so successful that within a few years the decision was made to integrate other elements of the countries' economies. In 1957, envisioning an "ever closer union," the Treaties of Rome created the European Economic Community (EEC) and the European Atomic Energy Community (Euratom), and the six member states undertook to eliminate trade barriers among themselves by forming a common market. In 1967, the institutions of all three communities were formally merged into the European Community (EC), creating a single Commission, a single Council of Ministers, and the body known today as the European Parliament. Members of the European Parliament were initially selected by national parliaments, but in 1979 the first direct elections were undertaken and have been held every five years since. In 1973, the first enlargement of the EC took place with the addition of Denmark, Ireland, and the UK. The 1980s saw further membership expansion with Greece joining in 1981 and Spain and Portugal in 1986. The 1992 Treaty of Maastricht laid the basis for further forms of cooperation in foreign and defense policy, in judicial and internal affairs, and in the creation of an economic and monetary union - including a common currency. This further integration created the European Union (EU), at the time standing alongside the EC. In 1995, Austria, Finland, and Sweden joined the EU/EC, raising the membership total to 15. A new currency, the euro, was launched in world money markets on 1 January 1999; it became the unit of exchange for all EU member states except Denmark, Sweden, and the UK. In 2002, citizens of those 12 countries began using euro banknotes and coins. Ten new countries joined the EU in 2004 - Cyprus, the Czech Republic, Estonia, Hungary, Latvia, Lithuania, Malta, Poland, Slovakia, and Slovenia. Bulgaria and Romania joined in 2007 and Croatia in 2013, bringing the current membership to 28. (Seven of these new countries - Cyprus, Estonia, Latvia, Lithuania, Malta, Slovakia, and Slovenia - have now adopted the euro, bringing total euro-zone membership to 19.) In an effort to ensure that the EU could function efficiently with an expanded membership, the Treaty of Nice (concluded in 2000; entered into force in 2003) set forth rules to streamline the size and procedures of EU institutions. An effort to establish a "Constitution for Europe," growing out of a Convention held in 2002-2003, foundered when it was rejected in referenda in France and the Netherlands in 2005. A subsequent effort in 2007 incorporated many of the features of the rejected draft Constitutional Treaty while also making a number of substantive and symbolic changes. The new treaty, referred to as the Treaty of Lisbon, sought to amend existing treaties rather than replace them. The treaty was approved at the EU intergovernmental conference of the then 27 member states held in Lisbon in December 2007, after which the process of national ratifications began. In October 2009, an Irish referendum approved the Lisbon Treaty (overturning a previous rejection) and cleared the way for an ultimate unanimous endorsement. Poland and the Czech Republic ratified soon after. The Lisbon Treaty came into force on 1 December 2009 and the EU officially replaced and succeeded the EC. The Treaty's provisions are part of the basic consolidated versions of the Treaty on European Union (TEU) and the Treaty on the Functioning of the European Union (TFEU) now governing what remains a very specific integration project. Frustrated by a remote bureaucracy in Brussels and massive migration into the country, UK citizens on 23 June 2016 narrowly voted to leave the EU. The so-called “Brexit” will take years to carry out, but could embolden skeptics of EU membership in other member states.