Discussed by philosophers for centuries, actively promoted from the 1920s onward by Count Richard Coudenhove-Kalergi’s Pan-European Movement, and officially proposed in 1929 by Aristide Briand on behalf of France, the idea of uniting Europe was revived again as World War II approached. In Britain a small private group that called itself Federal Union—in close touch with others at the Royal Institute of International Affairs (Chatham House)—began to campaign for unity in Europe as a last frail hope of preventing war. Some of the papers produced by its distinguished supporters, including work by Lord Lothian and Lionel Robbins, found their way to another group of activists in the Italian Resistance, led by, among others, Altiero Spinelli. One of the most stubborn of Mussolini’s political prisoners, he was freed in 1943 from confinement on an island off the coast between Rome and Naples. Admiring what he called “the clean, precise thinking of the English federalists,” he echoed it in the declaration he drafted for a secret grouping of Resistance leaders from eight other countries, including Germany. Britain thus contributed to Continental developments that British governments shunned for many years.

Support for European unity came from the right as well as the left, from liberals as well as dirigistes, from clerics as well as anticlericals, from “Atlanticists” as well as those who saw Europe as a “Third Force” between East and West. It even received official support, overt as well as implicit.

In 1939 the British Labour Party leader Clement Attlee declared: “Europe must federate or perish.” In 1940, prompted by Jean Monnet, Churchill’s government, in agreement with General de Gaulle, proposed a political union between Britain and France. In 1943 Churchill called for a Council of Europe after the war, and de Gaulle’s colleague René Mayer suggested an economic federation. In 1944 the exiled governments of Belgium, the Netherlands, and Luxembourg signed the Benelux Convention for a future customs union. Pope Pius XII, meanwhile, had envisaged a close union of European states.

Individual supporters of European unity included not only statesmen such as Paul-Henri Spaak from Belgium, Alcide De Gasperi from Italy, Robert Schuman from France, Johan Willem Beyen from the Netherlands, Konrad Adenauer from Germany, and Joseph Bech from Luxembourg but also such well-known writers as Albert Camus, Raymond Aron, George Orwell, Denis de Rougemont, and Ignazio Silone. All urged, and many helped to organize, what Winston Churchill called in 1946 “a kind of United States of Europe.”

In 1948 a number of activist organizations, coordinated by Joseph Retinger, former assistant to the late General Władysław Sikorski, head of the wartime Polish government-in-exile in London, staged a full-scale Congress of Europe in The Hague. Attended by 750 statesmen from throughout western Europe, including Spaak, De Gasperi, Churchill, Schuman, Adenauer, and a young French Resistance worker named François Mitterrand, it called for political and economic union, a European Assembly, and a European Court of Human Rights.

Some governments responded sympathetically. The postwar constitutions of France, West Germany, and Italy all envisaged limiting national sovereignty: the German text specifically looked forward to a united Europe. The British, however, were skeptical; and when in response to proposals by the French foreign minister Georges Bidault (who had attended The Hague Congress) the governments took action, it was of limited scope. In May 1949 they set up the Council of Europe, consisting of a Committee of Ministers and a Consultative Assembly.

To the activists, it was something; but it was not enough. The Council of Europe’s main achievement, apart from useful studies and discussions, was to produce the European Convention for the Protection of Human Rights (1950), effectively backed by a court and a commission. But the Consultative Assembly was just that: it had no power, and the Committee of Ministers had a veto.

The initiative to go further came from Monnet. His opportunity came when France was at loggerheads with Britain and the United States, both of which sought to remove the postwar restraints preventing German heavy industry from making its full contribution to the prosperity of the West. Monnet proposed to sidestep the dilemma by pooling coal and steel production in western Europe, including West Germany’s, under common institutions to replace with a light and shared rein the heavy control that the International Ruhr Authority had imposed on West Germany alone.

This was the essence of what became the Schuman Plan in 1950 when Robert Schuman, by then the French foreign minister, accepted it after Georges Bidault, the prime minister, had neglected to take it up. Its end product, initially embracing only six nations, was the formation of the European Coal and Steel Community, which began work in 1952.

Monnet and Schuman saw this as only a first step on the way to a European federation. Monnet followed it by proposing to René Pleven a similar solution to the problem of German rearmament: a European Defense Community. When that eventually failed, he proposed to Spaak and Beyen what became by 1957–58 the European Economic Community (EEC) and the European Atomic Energy Community (Euratom), a similar organization for the peaceful use of nuclear energy. The three institutions were ultimately merged to become the European Communities (EC) in 1967. With a Council of Ministers to make essential decisions (if need be by majority vote), a Commission to propose policy, and a European Parliament and Court of Justice to exert, respectively, legislative and judicial control, the EC had the embryo of a federal constitution, limited to economic and social affairs.

It also had the potential for crises, growth, and enlargement. Its first major crisis, indeed, concerned enlargement, when President de Gaulle vetoed the first British application to join, in 1963. The second crisis, two years later, was also provoked by de Gaulle, who objected to the extension of majority voting.

Weathering both crises, the EC proceeded to broaden its scope and to expand beyond the organization’s original six members—France, West Germany, Italy, Belgium, Luxembourg, the Netherlands. First came Britain, Ireland, and Denmark (1973), followed by Greece (1981) and then Spain and Portugal (1986). The Single European Act, promulgated July 1, 1987, built upon ongoing efforts at further social and economic cohesion and established a timetable for the completion of a common market. The Maastricht Treaty (Treaty on European Union), signed on February 7, 1992, created the European Union (EU), comprising three main components: a common foreign and security policy, an enhanced cooperation in domestic affairs, and the EC, renamed the European Community, which became the anchor of the EU with broader authority. Moreover, the treaty established EU citizenship, which allowed citizens, regardless of nationality, to vote and run for office in elections in the country of their residence both for local office and for the organization’s increasingly important legislative body, the European Parliament. Enthusiasm for the EU in the member countries was not universal (it took two referendums for Danish voters to approve their country’s involvement, and the referendum on membership was barely approved by the French electorate), but the treaty officially took effect on November 1, 1993. ‘‘Convergence criteria’’ (relating to levels of government spending, inflation, public debt, and exchange-rate stability) were established for participation in a common European currency. Although some countries failed to qualify (Greece) or chose to remain outside the system (Britain, Denmark, and Sweden, the last having joined the EU in 1995 along with Austria and Finland), 11 countries on January 1, 1999, relinquished control over their exchange rates and began a transition to the replacement of their national currencies with a single monetary unit, the euro.

Meanwhile, members of the EU began acting collectively in foreign policy, notably attempting to bring peace to the countries of the former federation of Yugoslavia, which splintered violently, beginning with the secession of Croatia and Slovenia in 1991 and that of Macedonia (later called North Macedonia) in 1992. Ethnic civil war was more protracted in Bosnia and Herzegovina (sparking UN intervention), and EU members helped halt the fighting there through participation in the brokering of the Dayton Accords. Acting under the aegis of NATO, a number of EU countries intervened militarily in the conflict in Kosovo (1998–99), between that region’s ethnic Albanian majority and Serb minority and the government of the rump Yugoslavian state (comprising Serbia and Montenegro), and again in Afghanistan, where the Taliban regime had provided a home for the radical Islamists responsible for the September 11 attacks on the United States in 2001. Terrorist bombings by Islamist radicals also struck Europe, in Madrid in March 2004 and in London in July 2005.

In 2004 the EU admitted 10 more countries: the Czech Republic and Slovakia (independent states formed by the dissolution of Czechoslovakia in 1992), as well as Cyprus, Estonia, Hungary, Latvia, Lithuania, Malta, Poland, and Slovenia. Bulgaria and Romania joined in 2007. Other than Cyprus and Malta, the 21st-century additions were all former members of the communist bloc. To the consternation of Russia, many of them also joined NATO. In 2008 Russia showed its support for Serbia (a solitary country after the split in 2006 of Serbia and Montenegro, the renamed loose successor state to Yugoslavia), when Kosovo declared its independence despite Serbian opposition. Russia’s relations with the EU and NATO, particularly those organizations’ former communist members, were sometimes tense, as when Russia objected strenuously to U.S. plans to deploy a missile-defense radar system in the Czech Republic and Poland.

Efforts begun in 2002 to draft a constitution for the enlarged EU led to the signing of a document in 2004 that failed to be ratified by French and Dutch voters in 2005. A reform version of the failed constitution, stewarded by Germany, resulted in the Lisbon Treaty of December 2007; however, it too was scuttled, at least temporarily, when Irish voters rejected it in 2008. In October 2009 Ireland approved the treaty in a second referendum, and Poland completed its ratification process as well. That November the Czech Republic became the final country to ratify the treaty, which entered into force on December 1, 2009.

The EC was founded in response to a checkered half-century of European history, when some of the world’s most civilized countries had plumbed depths of savagery, folly, tyranny, and genocide that in a work of science fiction would be hard to believe. The EC’s most obvious purpose had been to reconcile former enemies and prevent war. Its second aim was to avoid the economic errors Europeans had made in the 1930s, when, instead of collaborating on a global recovery policy in response to the Great Depression, they worsened the crisis through economic nationalism that was the breeding ground for dictatorship. Moreover, Europe’s individual countries, once great powers, were dwarfed numerically, politically, and militarily by the United States and the Soviet Union (until its dissolution in 1991–92) and economically by the United States and Japan. By the early 21st century the gigantic countries of India and China had also become economic rivals for the European Union and for a Europe that increasingly saw greater cohesiveness as the path not only to holding its own vis-à-vis political and economic superpowers but also to maximizing its power to meet its wider responsibilities in the world.