The EU traces its origins to the European Coal and Steel Community (ECSC) and the European Economic Community (EEC), established, respectively, by the 1951 Treaty of Paris and 1957 Treaty of Rome. The original members of what came to be known as the European Communities were the Inner Six: Belgium, France, Italy, Luxembourg, the Netherlands, and West Germany. The Communities and its successors have grown in size by the accession of new member states and in power by the addition of policy areas to its remit. While no member state has left the EU or its predecessors, the United Kingdom signified an intention to leave after a membership referendum in June 2016 and is negotiating its withdrawal. The Maastricht Treaty established the European Union in 1993 and introduced European citizenship. The latest major amendment to the constitutional basis of the EU, the Treaty of Lisbon, came into force in 2009.
The European Union provides more foreign aid than any other economic union. Covering 7.3% of the world population, the EU in 2017 generated a nominal gross domestic product (GDP) of 19.670 trillion US dollars, constituting approximately 24.6% of global nominal GDP and 16.5% when measured in terms of purchasing power parity. Additionally, 27 out of 28 EU countries have a very high Human Development Index, according to the United Nations Development Programme. In 2012, the EU was awarded the Nobel Peace Prize. Through the Common Foreign and Security Policy, the EU has developed a role in external relations and defence. The union maintains permanent diplomatic missions throughout the world and represents itself at the United Nations, the World Trade Organization, the G7 and the G20. Because of its global influence, the European Union has been described as an emerging superpower.
During the centuries following the fall of Rome in 476, several European States viewed themselves as translatio imperii of the defunct Roman Empire: the Frankish Empire (481–843) and the Holy Roman Empire (962–1806) were attempts to resurrect Rome in the West. The Russian Tsardom, and ultimately the Empire (1547–1917), declared Moscow to be Third Rome and inheritor of the Eastern tradition after the fall of Constantinople in 1453. The gap between Greek East and Latin West had already been widened by the political scission of the Roman Empire in the 4th century and the Great Schism of 1054; and would be eventually widened again by the Iron Curtain (1945–91). The Medieval Christendom and political power of the Papacy are also often cited as premisses to European integration and unity.
Pan-European political thought truly emerged during the 19th century, inspired by the liberal ideas of the French and American Revolutions after the demise of Napoléon's Empire (1804–15). In the decades following the outcomes of the Congress of Vienna, ideals of European unity flourished across the continent, especially in the writings of Wojciech Jastrzębowski, Giuseppe Mazzini or Theodore de Korwin Szymanowski. The term United States of Europe (French: États-Unis d'Europe) was famously used at that time by Victor Hugo during a speech at the International Peace Congress held in Paris in 1849.
During the interwar period, the consciousness that national markets in Europe were interdependent though confrontational, along with the observation of a larger and growing US market on the other side of the ocean, nourished the urge for the economic integration of the continent. In 1920, advocating the creation of a European economic union, British economist John Maynard Keynes wrote that "a Free Trade Union should be established ... to impose no protectionist tariffs whatever against the produce of other members of the Union." During the same decade, Richard von Coudenhove-Kalergi, one of the first to imagine of a modern political union of Europe, founded the Pan-Europa Movement. His ideas influenced his contemporaries, among which then Prime Minister of France Aristide Briand. In 1929, the later gave a famous speech in favour of a European Union before the assembly of the League of Nations, precursor of the United Nations.
After World War II, European integration was seen as an antidote to the extreme nationalism which had devastated the continent. In a speech delivered on 19 September 1946 at the University of Zürich, Switzerland, Winston Churchill postulated the emerging of a United States of Europe during the 20th century. The 1948 Hague Congress was a pivotal moment in European federal history, as it led to the creation of the European Movement International and of the College of Europe, where Europe's future leaders would live and study together. 1952 saw the creation of the European Coal and Steel Community, which was declared to be "a first step in the federation of Europe." The supporters of the Community included Alcide De Gasperi, Jean Monnet, Robert Schuman, and Paul-Henri Spaak. These men and others are officially credited as the Founding fathers of the European Union.