The Act of Union 1800 added the Kingdom of Ireland to create the United Kingdom of Great Britain and Ireland.
The first decades were marked by Jacobite risings which ended with defeat for the Stuart cause at Culloden in 1746. In 1763, victory in the Seven Years' War led to the growth of the First British Empire. With the defeat by the United States, France and Spain in the War of American Independence, Britain lost its 13 American colonies and rebuilt a Second British Empire based in Asia and Africa. As a result, British culture, and its technological, political, constitutional, and linguistic influence, became worldwide. Politically, the central event was the French Revolution and its Napoleonic aftermath from 1793 to 1815, which British elites saw as a profound threat, and worked energetically to form multiple coalitions that finally defeated Napoleon in 1815. The Tories, who came to power in 1783, remained in power (with a short interruption) until 1830. Forces of reform, often emanating from the Evangelical religious elements, opened decades of political reform that broadened the ballot, and opened the economy to free trade. The outstanding political leaders of the 19th century included Palmerston, Disraeli, Gladstone, and Salisbury. Culturally the Victorian era (Queen Victoria reigned 1837–1901) was a time of prosperity and dominant middle-class virtues when Britain dominated the world economy and maintained a generally peaceful century, 1815–1914. The First World War (1914–1918), in alliance with France, Russia and the United States, was a furious but ultimately successful total war with Germany. The resulting League of Nations was a favourite project in Interwar Britain. However, while the Empire remained strong, as did the London financial markets, the British industrial base began to slip behind Germany and especially the United States. Sentiments for peace were so strong that the nation supported appeasement of Hitler's Germany in the late 1930s, until the Nazi invasion of Poland in 1939 opened the Second World War. In the Second World War 1939–45, France, the Soviet Union the U.S. joined Britain as the main Allied powers.
Britain was no longer a military or economic superpower, as seen in the Suez Crisis of 1956. Britain no longer had the wealth to maintain an empire, so it granted independence to almost all its possessions. The new states typically joined the Commonwealth of Nations. Postwar years saw great hardships, alleviated somewhat by large-scale financial aid from the United States, and some from Canada. Prosperity returned in the 1950s. Meanwhile, in 1945–50 the Labour Party built a welfare state, nationalized many industries, and created the National Health Service. The UK took a strong stand against Communist expansion after 1945, playing a major role in the Cold War and the formation of NATO as an anti-Soviet military alliance with West Germany, France, the U.S., Canada and smaller countries. NATO remains a powerful military coalition. The UK has been a leading member of the United Nations since its founding, as well as numerous other international organizations. In the 1990s, neoliberalism led to the privatisation of nationalized industries and significant deregulation of business affairs. London's status as a world financial hub grew continuously. Since the 1990s large-scale devolution movements in Northern Ireland, Scotland and Wales have decentralized political decision-making. Britain has wobbled back and forth on its economic relationships with Western Europe. It joined the European Union in 1973, thereby weakening economic ties with its Commonwealth. However, the Brexit referendum in 2016 committed the UK to an exit from the European Union; negotiations are currently underway.
In 1922, Catholic Ireland seceded to become the Irish Free State; a day later, Northern Ireland seceded from the Free State and returned to the United Kingdom. In 1927 the United Kingdom changed its formal title to the United Kingdom of Great Britain and Northern Ireland,[3] usually shortened to Britain and (after 1945) to the United Kingdom or UK.