The Maastricht Treaty, which led to the formation of the European Union, was signed by the member states of the European Communities.

The Treaty on European Union, commonly known as the Maastricht Treaty, is the foundation treaty of the European Union (EU). Concluded in 1992 between the then-twelve member states of the European Communities, it announced "a new stage in the process of European integration" chiefly in provisions for a shared European citizenship, for the eventual introduction of a single currency, and for common foreign and security policies, and a number of changes to the European institutions and their decision taking procedures, not least a strengthening of the powers of the European Parliament and more majority voting on the Council of Ministers. Although these were seen by many to presage a "federal Europe", key areas remained inter-governmental with national governments collectively taking key decisions. This constitutional debate continued through the negotiation of subsequent treaties, culminating in the 2007 Treaty of Lisbon.

The European Union (EU) is a supranational political and economic union of 27 member states that are located primarily in Europe. The union has a total area of 4,233,255 km² (1,634,469 sq mi) and an estimated population of over 450 million as of 2025. The EU is often described as a sui generis political entity combining characteristics of both a federation and a confederation.

The European Communities (EC) were three international organizations that were governed by the same set of institutions. These were the European Coal and Steel Community (ECSC), the European Atomic Energy Community, and the European Economic Community (EEC), the last of which was renamed the European Community (EC) in 1993 by the Maastricht Treaty establishing the European Union. The European Union was established at that time more as a concept rather than an entity, while the Communities remained the actual subjects of international law impersonating the rather abstract Union, becoming at the same time its first pillar. In popular language, however, the singular European Community was sometimes used interchangeably with the plural phrase, in the sense of referring to all three entities.