Merger Treaty: The European Community is formally created out of a merger between the Common Market, the European Coal and Steel Community, and the European Atomic Energy Commission.

The Merger Treaty, also known as the Treaty of Brussels, was a European treaty which unified the executive institutions of the European Coal and Steel Community (ECSC), European Atomic Energy Community (Euratom) and the European Economic Community (EEC). The treaty was signed in Brussels on 8 April 1965 and came into force on 1 July 1967. It set out that the Commission of the European Communities should replace the High Authority of the ECSC, the Commission of the EEC and the Commission of Euratom, and that the Council of the European Communities should replace the Special Council of Ministers of the ECSC, the Council of the EEC and the Council of Euratom. Although each Community remained legally independent, they shared common institutions and were together known as the European Communities. This treaty is regarded by some as the real beginning of the modern European Union.

The European Economic Community (EEC) was a regional organisation created by the Treaty of Rome of 1957, aiming to foster economic integration among its member states. It was subsequently renamed the European Community (EC) upon becoming integrated into the first pillar of the newly formed European Union (EU) in 1993. In the popular language, the singular European Community was sometimes inaccurately used in the wider sense of the plural European Communities, in spite of the latter designation covering all the three constituent entities of the first pillar. The EEC was also known as the European Common Market (ECM) in the English-speaking countries, and sometimes referred to as the European Community even before it was officially renamed as such in 1993. In 2009, the EC formally ceased to exist and its institutions were directly absorbed by the EU. This made the Union the formal successor institution of the Community.

The European Coal and Steel Community (ECSC) was a European organization created after World War II to integrate Europe's coal and steel industries into a single common market based on the principle of supranationalism which would be governed by the creation of a High Authority made up of appointed representatives from the member states who would not represent their national interest, but would take and make decisions in the general interests of the Community as a whole. It was formally established in 1951 by the Treaty of Paris, signed by Belgium, France, Italy, Luxembourg, the Netherlands, and West Germany and was generally seen as the first step in the process of European integration following the end of the Second World War in Europe. The organization's subsequent enlargement of both members and duties ultimately led to the creation of the European Union.