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.
Ireland
In December 1921, the Anglo-Irish Treaty was concluded between the British government and representatives of the Second Dáil. It gave Ireland complete independence in its home affairs and practical independence for foreign policy, but an opt-out clause allowed Northern Ireland to remain within the United Kingdom, which it immediately exercised as expected. Additionally, Members of the Free State Parliament were required to swear an oath of allegiance to the Constitution of the Irish Free State and make a statement of faithfulness to the King. Disagreements over these provisions led to a split in the nationalist movement and a subsequent Irish Civil War between the new government of the Irish Free State and those opposed to the treaty, led by Éamon de Valera. The civil war officially ended in May 1923 when de Valera issued a cease-fire order.