The European Union (EU) is a political and economic union of 28 member states that are located primarily in Europe. It has an area of 4,475,757 km2 (1,728,099 sq mi) and an estimated population of about 513 million. The EU has developed an internal single market through a standardised system of laws that apply in all member states in those matters, and only those matters, where members have agreed to act as one. EU policies aim to ensure the free movement of people, goods, services and capital within the internal market, enact legislation in justice and home affairs and maintain common policies on trade, agriculture, fisheries and regional development. For travel within the Schengen Area, passport controls have been abolished. A monetary union was established in 1999 and came into full force in 2002 and is composed of 19 EU member states which use the euro currency. The EU and European citizenship were established when the Maastricht Treaty came into force in 1993. 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 their successors have grown in size by the accession of new member states and in power by the addition of policy areas to their remit. The latest major amendment to the constitutional basis of the EU, the Treaty of Lisbon, came into force in 2009. No member state has left the EU or its antecedent organisations. (Greenland, an autonomous country within Denmark, left the Communities in 1985). However, the United Kingdom signified its intention to leave after a membership referendum in June 2016 and is negotiating its withdrawal. Containing 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. Additionally, all 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 was described in 2006 as an emerging superpower. During the centuries following the fall of Rome in 476, several European States viewed themselves as translatio imperii ('transfer of rule') of the defunct Roman Empire: the Frankish Empire (481–843) and the Holy Roman Empire (962–1806) were thereby attempts to resurrect Rome in the West. This political philosophy of a supra-national rule over the continent, similar to the example of the ancient Roman Empire, resulted in the early Middle Ages in the concept of a renovatio imperii ('restoration of the empire'), either in the forms of the Reichsidee ('imperial idea') or the religiously inspired Imperium Christianum ('christian empire'). Medieval Christendom and the political power of the Papacy are often cited as conducive to European integration and unity. In the oriental parts of the continent, 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). 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 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 latter gave a speech in favour of a European Union before the assembly of the League of Nations, precursor of the United Nations. In a radio address in March 1943, with war still raging, Britain's leader Sir Winston Churchill spoke warmly of 'restoring the true greatness of Europe' once victory had been achieved, and mused on the post-war creation of a 'Council of Europe' which would bring the European nations together to build peace.