The Second World War and the devastation that Europe suffered brought an idea of uniting European countries in an area in which cooperation was seen as possible at the time. Therefore, the European Coal and Steal Community (ECSC) was firstly set up in 1950, thus marking the very beginning of the European integration process. The target area was economic cooperation in coal and steel as an important industry for military supplies, thus aiming at providing perennial peace. It was in particular a major concern for France, who wanted to make sure that a similar confrontation with neighbours never occurred again, and for Germany, who carried the negative burden from the recent war.
The six founding countries of the ECSC were Belgium, France, Germany, Italy, Luxembourg and the Netherlands. The Treaty of Rome in 1957 established the European Economic Community (EEC) aiming to create a common market between these member states, which culminated in an economic boom during the 1960s. Those trends towards deeper economic cooperation were aided by the elimination of customs duties in trade. The Common Agricultural Policy was also negotiated, resulting in joint control over food production. The result that followed was a surplus in production of agricultural products.
From the six founding countries the EEC grew to nine in 1973, when Denmark, Ireland and the United Kingdom joined. Six years later, in 1979, the European Parliament increased its influence the when first direct elections were held. Greece joined the EEC in 1981, followed by Spain and Portugal in 1986, thus bringing the community to the membership of twelve. There were significant changes and results achieved in 1990s - the Single Market was completed and laid down foundations for the free movement of goods, services, people and capital between the member states. The European Union was formally established with the ratification of the Treaty of Maastricht and new areas were assigned to the EU policy agenda, such as environment and common security. In 1995 three more countries – Austria, Finland and Sweden – joined the Union, thus increasing its total membership to fifteen. In the same period the Schengen Agreement became part of the EU law as part of the Treaty of Amsterdam, paving the way for passport-free travel in an EU without internal borders.
In January 2002 the common currency Euro was introduced replacing previous national currencies in twelve member states (the UK, Sweden and Denmark opted to stay out of the Eurozone). In 2004 the EU saw the largest, so-called ‘Eastern enlargement’ incorporating ten countries: Cyprus, the Czech Republic, Estonia, Hungary, Latvia, Lithuania, Malta, Poland, Slovakia and Slovenia. Following the enlargement of 2007 when Bulgaria and Romania joined, the last enlargement was in July 2013 with the accession of Croatia. Five of the new EU member states have so far joined the Eurozone as well: Cyprus, Estonia, Malta, Slovakia and Slovenia.
The Treaty establishing a Constitution for Europe, commonly referred to as the European Constitution, was signed in Rome in 2004, with the aim of replacing all previous treaties with one single document. However, it was rejected by French and Dutch voters and was never implemented. A new reform treaty, the Lisbon Treaty, was signed in December 2007 and came into force on 1 December 2009 after being ratified by all Member States. It introduced inter alia the single legal personality for the EU, the President of the European Council and the High Representative of the Union for Foreign Affairs and Security Policy. It also brought about the extension of the co-decision procedure and gave the EU Charter of Fundamental Rights the same legal basis as the treaties.
You can read more on the history of the EU here.