Belgium, France, Germany, Italy, Luxembourg and Netherlands originally formed the European Union. In 1973, Denmark, Ireland and the United Kingdom joined. By 1995, Austria, Finland, Sweden, Portugal, Spain and Greece had become members. Cyprus, Czech Republic, Estonia, Hungary, Latvia, Lithuania, Malta, Poland, Slovakia, Slovenia, Croatia, Bulgaria and Romania have joined since 2003.
As of April 2015, the following five countries were in the process of becoming members of the European Union (EU): Albania, Iceland, Montenegro, Serbia, Turkey and Macedonia. A unanimous decision by the EU Council is required before a country can begin the process of becoming a member. Becoming a member of the EU requires that a country meet the "Copenhagen criteria," which includes having a free-market economy, a stable democracy including the rule of law and acceptance of all EU legislation.
The EU was created by the original six countries to foster economic cooperation after the Second World War. The theory was that the countries would trade with each other and become economically interdependent, and therefore more likely to avoid conflict. The organization has evolved into multiple policy areas including development aid and environmental controls. The euro, the common currency used in 19 of the 28 EU countries, was launched in 1999.