The Troubles
The Government of Ireland Act 1920 partitioned the island of Ireland into two separate jurisdictions, Southern Ireland and Northern Ireland, both devolved regions of the United Kingdom. This partition of Ireland was confirmed when the Parliament of Northern Ireland exercised its right in December 1922 under the Anglo-Irish Treaty of 1921 to ""opt out"" of the newly established Irish Free State. A part of the treaty signed in 1922 mandated that a boundary commission would sit to decide where the frontier of the northern state would be in relation to its southern neighbour. After the Irish Civil War of 1922–1923, this part of the treaty was given less priority by the new Dublin government led by W. T. Cosgrave, and was quietly dropped. As counties Fermanagh and Tyrone and border areas of Londonderry, Armagh, and Down were mainly nationalist, the Irish Boundary Commission could reduce Northern Ireland to four counties or less.
History of the United Kingdom
In 1912, the House of Commons passed a new Home Rule bill. Under the 1911 Parliament Act the House of Lords retained the power to delay legislation by up to two years, so it was eventually enacted as the Government of Ireland Act 1914, but suspended for the duration of the war. Civil war threatened when the Protestant-Unionists of Northern Ireland refused to be placed under Catholic-Nationalist control. Semi-military units were formed ready to fight—the Unionist Ulster Volunteers opposed to the Act and their Nationalist counterparts, the Irish Volunteers supporting the Act. The outbreak of the World War in 1914 put the crisis on political hold. A disorganized Easter Rising in 1916 was brutally suppressed by the British, which had the effect of galvanizing Nationalist demands for independence. Prime Minister Lloyd George failed to introduce Home Rule in 1918 and in the December 1918 General Election Sinn Féin won a majority of Irish seats. Its MPs refused to take their seats at Westminster, instead choosing to sit in the First Dáil parliament in Dublin. A declaration of independence was ratified by Dáil Éireann, the self-declared Republic's parliament in January 1919. An Anglo-Irish War was fought between Crown forces and the Irish Republican Army between January 1919 and June 1921. The war ended with the Anglo-Irish Treaty of December 1921 that established the Irish Free State. Six northern, predominantly Protestant counties became Northern Ireland and have remained part of the United Kingdom ever since, despite demands of the Catholic minority to unite with the Republic of Ireland. Britain officially adopted the name "United Kingdom of Great Britain and Northern Ireland" by the Royal and Parliamentary Titles Act 1927.