The European Union (EC), is officially created as the Maastricht Treaty, signed on February 7, 1992, becomes effective; establishing the beginning of a single market with free movement for people, goods, services and money.
The initial 12 EU member states were those that had ratified the Maastricht Treaty. These 12 nations were: Belgium, Denmark, Germany, France, Greece, Ireland, Italy, Luxembourg, the Netherlands, Portugal, Spain, and the United Kingdom. (The UK formally left the EU effective on January 31, 2020) .
Note - The six countries who were the initial signatories of the Treaty of Paris (1951) and the Treaty of Rome (1957), which laid the groundwork for the EU's predecessors and the sharing of sovereignty among member states were: Belgium, France, Germany, Italy, Luxembourg and the Netherlands. These countries are referred to as EU “Founding Members” and also the "Inner Six".
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