Republic of Ireland–United Kingdom border
Originally intended as an internal boundary within the United Kingdom of Great Britain and Ireland, the border was created in 1921 under the United Kingdom Parliament's Government of Ireland Act 1920. Prior to this, a separatist Irish parliament had been established in Dublin, which did not recognise the Government of Ireland Act, and was actively engaged in the Irish War of Independence. The Act was intended to deliver Home Rule in Ireland, with separate parliaments for Southern Ireland (which included three of the nine counties of Ulster) and Northern Ireland. Six of the thirty-two counties of Ireland were assigned to Northern Ireland, and the rest of Ireland comprising 26 counties to Southern Ireland.
Northern Ireland
Events overtook the government. The pro-independence Sinn Féin won 73 of the 105 parliamentary seats in Ireland at the general election of 1918, and unilaterally established the First Dáil, an extrajudicial parliament in Ireland. Ireland was partitioned between Northern Ireland and Southern Ireland in 1921, under the terms of Lloyd George's Government of Ireland Act 1920, during the Anglo-Irish War between Irish republican and British forces. A truce was established on 11 July; the war ended on 6 December 1921 with the signing of the Anglo-Irish Treaty, which created the Irish Free State. Under the terms of the treaty, Northern Ireland would become part of the Free State unless the government opted out by presenting an address to the king, although in practice partition remained in place.