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.