On 1 May 1707 the united Kingdom of Great Britain was created by the political union of the Kingdom of England and the Kingdom of Scotland. This was the result of the Treaty of Union, agreed on 22 July 1706, and then ratified by both the Parliament of England and the Parliament of Scotland – each passing an Act of Union in 1707.The kingdoms of England and Scotland, together with the kingdom of Ireland, had already been in a personal union as a result of the Union of the Crowns in 1603, when James VI, King of Scots inherited the Kingdoms of England and Ireland and moved his court from Edinburgh to London (Wales had previously been added to the English crown by conquest by the 13th century, and fully incorporated into the Kingdom of England by statute in the 16th century. Ireland had been partially conquered following the Anglo-Norman invasion in the 12th century and had been constituted as a kingdom in personal union with the English crown in 1542). Until 1707 England, Scotland and Ireland remained separate political entities and retained their separate political institutions. Almost a century later the Kingdom of Ireland and the Kingdom of Great Britain were united to form the United Kingdom of Great Britain and Ireland with the passing of the Acts of Union 1800.In this way, the United Kingdom became a single kingdom. Disputes within Ireland over the terms of Irish Home Rule eventually led to the partition of the island in 1921, with Dominion status for the Irish Free State in 1922, while Northern Ireland remained part of the United Kingdom. As a result the formal title of the United Kingdom was changed to its current form in 1927; the United Kingdom of Great Britain and Northern Ireland.
In the 18th century the United Kingdom played an important role in developing Western ideas of the parliamentary system, as well as making significant contributions to literature, the arts and science. The UK-led Industrial Revolution transformed the country and fuelled the growing British Empire. During this time the UK, like other great powers, was involved in colonial exploitation, including the Atlantic slave trade, although with the passing of the Slave Trade Act in 1807 the UK took a leading role in combating the trade in slaves.
After the defeat of France in the Revolutionary and Napoleonic Wars (1792–1815) the UK emerged as the principal naval and economic power of the 19th century (with London the largest city in the world from about 1830 to 1930) and remained a foremost power into the mid 20th century.
The UK, along with Russia, France and (after 1917) the USA, was one of the major powers opposing Germany and its allies in World War I (1914–18). The UK armed forces grew to over five million people engaged across much of its empire, several regions in Europe and increasingly took a major role on the Western front. The nation suffered an estimated two and a half million casualties and finished the war with a huge national debt. After the war the United Kingdom received the League of Nations mandate over former German and Ottoman colonies and the British Empire had expanded to its greatest extent, covering a fifth of the world's land surface and a quarter of its population. The Great Depression (1929–32) occurred at a time when the UK was still far from having recovered from the effects of the war and led to hardship as well as political and social unrest.
The United Kingdom was one of the three main Allies of World War II (known as the "Big Three"). Following the defeat of its European allies, in the first year of the war, the United Kingdom continued the fight against Germany which took form with notable periods such as the Battle of Britain and the Battle of the Atlantic. Following the D-Day landings and the subsequent defeat of Germany and her allies, the UK was one of the "Big Three" that met at the end of the war to plan the postwar world. The war left the United Kingdom financially damaged although aid from the Marshall Plan and loans taken from both the United States and Canada helped the UK on the road to recovery.
The immediate postwar years saw the establishment of the Welfare State, including comprehensive public health services. As a result of a shortage of workers initial postwar policy was to bring in workers from Germany, Poland and other European countries. The Colonial office, however, persuaded the British Government that it should offer employment to British subjects of the Commonwealth, creating a multiethnic Britain. Although the new postwar limits of Britain's political role were confirmed by the Suez Crisis of 1956, the international spread of the English language meant the continuing influence of its literature and culture while, from the 1960s, its popular culture also found influence abroad. Following a period of global economic slowdown and industrial strife in the 1970s the inflow of substantial North Sea oil revenues and economic growth began in 1984.
Inequalities between the Protestant and Catholic groups in Northern Ireland, combined with fears among unionists of the claim by the Republic of Ireland to the whole island, led to a breakout of violence in 1966.Paramilitary groups were created by both sides and, after riots in Derry in 1969, the British Army was called in to try to maintain peace. On 24 March 1972 the Parliament of Northern Ireland was suspended and Direct Rule was introduced from London. Eventually the Anglo-Irish Agreement was signed in November 1985, in which the Republic of Ireland acknowledged the United Kingdom's rule in the North in exchange for some say in governance. Negotiations eventually led to the 1998 Belfast Agreement between the United Kingdom and the Republic of Ireland, reflecting the terms of a peace settlement supported by most of the main political parties. The Agreement, approved by referendums in both parts of Ireland, created a new Northern Ireland Assembly and a power-sharing executive. The Constitution of Ireland was amended to replace a claim it made to the territory of Northern Ireland, while also acknowledging the nationalist desire for a united Ireland. The IRA and most other armed organisations ended their activities and destroyed their weaponry.
The United Kingdom was one of the 12 founding members of the European Union (EU) with the signing of the Maastricht Treaty in 1992. Prior to that it had been a member of the EU's forerunner, the European Economic Community (EEC), from 1973. The end of the 20th century saw major changes to the governance of the UK with the establishment of devolved national administrations for Northern Ireland, Scotland, and Wales following pre-legislative referendums.