stylistic:tone
Postwar 20th century

The UK was one of the Big Three powers (along with the US and the Soviet Union) who met to plan the post-war world; it drafted the Declaration by United Nations with the United States and became one of the five permanent members of the United Nations Security Council. It worked closely with the United States to establish the IMF, World Bank and NATO. The war left the UK severely weakened and financially dependent on the Marshall Plan, but it was spared the total war that devastated eastern Europe.

In the immediate post-war years, the Labour government under Clement Attlee initiated a radical programme of reforms, which significantly impacted British society in the following decades. Major industries and public utilities were nationalised, a welfare state was established, and a comprehensive, publicly funded healthcare system, the National Health Service, was created. The rise of nationalism in the colonies coincided with Britain's much-diminished economic position, so that a policy of decolonisation was unavoidable. Independence was granted to India and Pakistan in 1947. Over the next three decades, most colonies of the British Empire gained their independence, and many became members of the Commonwealth of Nations.

The UK was the third country to develop a nuclear weapons arsenal (with its first atomic bomb test, Operation Hurricane, in 1952), but the post-war limits of Britain's international role were illustrated by the Suez Crisis of 1956. The international spread of the English language ensured the continuing international influence of its literature and culture. As a result of a shortage of workers in the 1950s, the government encouraged immigration from Commonwealth countries. In the following decades, the UK became a more multi-ethnic society. Despite rising living standards in the late 1950s and 1960s, the UK's economic performance was less successful than many of its main competitors such as France, West Germany and Japan. The UK was the first democratic nation to lower its voting age to 18 in 1969.

In the decades-long process of European integration, the UK was a founding member of the Western European Union, established with the London and Paris Conferences in 1954. In 1960 the UK was one of the seven founding members of the European Free Trade Association (EFTA), but in 1973 it left to join the European Communities (EC). In a 1975 referendum 67% voted to stay in it. When the EC became the European Union (EU) in 1992, the UK was one of the 12 founding member states.

From the late 1960s, Northern Ireland suffered communal and paramilitary violence (sometimes affecting other parts of the UK) conventionally known as the Troubles. It is usually considered to have ended with the 1998 Belfast "Good Friday" Agreement. Following a period of widespread economic slowdown and industrial strife in the 1970s, the Conservative government of the 1980s led by Margaret Thatcher initiated a radical policy of monetarism, deregulation, particularly of the financial sector (for example, the Big Bang in 1986) and labour markets, the sale of state-owned companies (privatisation), and the withdrawal of subsidies to others.

In 1982, Argentina invaded the British territories of South Georgia and the Falkland Islands, leading to the 10-week Falklands War in which Argentine forces were defeated. The inhabitants of the islands are predominantly descendants of British settlers, and strongly favour British sovereignty, expressed in a 2013 referendum. From 1984, the UK economy was helped by the inflow of substantial North Sea oil revenues. Another British overseas territory, Gibraltar, ceded to Great Britain in the 1713 Treaty of Utrecht, is a key military base. A referendum in 2002 on shared sovereignty with Spain was rejected by 98.97% of voters in the territory.

Around the end of the 20th century, there were major changes to the governance of the UK with the establishment of devolved administrations for Scotland, Wales and Northern Ireland. The statutory incorporation followed acceptance of the European Convention on Human Rights. The UK remained a great power with global diplomatic and military influence and a leading role in the United Nations and NATO.