British unionism is a political ideology favoring the continuation of the United Kingdom. Unionism supports the union of England, Scotland, Wales, and Northern Ireland, and it historically supported the union of Ireland with the UK as well. During the 19th and early 20th centuries, unionism was a major issue in Ireland, where the Irish Unionist Alliance called for a continuation of the union, while Sinn Fein and other left-wing nationalist parties called for home rule or independence. In 1922, most of Ireland gained independence from Britain, but Northern Ireland remained under British control, ostensibly so that the British could protect its Protestant Ulster Scots and Anglo-Irish citizens. As a result, the ideology of Ulster unionism developed, opposing Irish republicanism.
British unionism has since been a major issue in Scotland and, to a lesser degree, Wales. In Scotland, several referendums were held on home rule and independence, leading to the creation of a Scottish Parliament in 1999. In 2014, an independence referendum failed by a narrow margin, with 44.7% supporting independence and 55.3% opposing. The UK Conservative Party, UK Labor Party, UK Liberal Democrats, and the UK Independence Party all support British unionism, and it is local parties such as Scotland's Scottish National Party and Wales' Plaid Cymru that advocate independence.