In 1912 the House of Commons passed a new Home Rule bill. Under the Parliament Act 1911 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.