United Kingdom, island country located off the northwestern coast of mainland Europe. The United Kingdom comprises the whole of the island of Great Britain—which contains England, Wales, and Scotland—as well as the northern portion of the island of Ireland. The name Britain is sometimes used to refer to the United Kingdom as a whole. The capital is London, which is among the world’s leading commercial, financial, and cultural centres.
Other major cities include Birmingham, Liverpool, and Manchester in England, Belfast and Londonderry in Northern Ireland, Edinburgh and Glasgow in Scotland, and Swansea and Cardiff in Wales. The origins of the United Kingdom can be traced to the time of the Anglo-Saxon king Athelstan, who in the early 10th century CE secured the allegiance of neighbouring Celtic kingdoms and became “the first to rule what previously many kings shared between them,” in the words of a contemporary chronicle.
Through subsequent conquest over the following centuries, kingdoms lying farther afield came under English dominion. Wales, a congeries of Celtic kingdoms lying in Great Britain’s southwest, was formally united with England by the Acts of Union of 1536 and 1542. Scotland, ruled from London since 1603, formally was joined with England and Wales in 1707 to form the United Kingdom of Great Britain. (The adjective “British” came into use at this time to refer to all the kingdom’s peoples.) Ireland came under English control during the 1600s and was formally united with Great Britain through the Act of Union of 1800. The republic of Ireland gained its independence in 1922, but six of Ulster’s nine counties remained part of the United Kingdom as Northern Ireland.
Relations between these constituent states and England have been marked by controversy and, at times, open rebellion and even warfare. These tensions relaxed somewhat during the late 20th century, when devolved assemblies were introduced in Northern Ireland, Scotland, and Wales. Nonetheless, even with the establishment of a power-sharing assembly after referenda in both Northern Ireland and the Irish republic, relations between Northern Ireland’s unionists (who favour continued British sovereignty over Northern Ireland) and nationalists (who favour unification with the republic of Ireland) remained tense into the 21st century.