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.
Partition of Ireland
Following the start of the Troubles in Northern Ireland in 1969, the Sunningdale Agreement was signed by the Irish and British governments in 1973. This collapsed in May 1974 due to the Ulster Workers' Council strike, and the new British Prime Minister Harold Wilson considered a rapid withdrawal of the British Army and administration from Northern Ireland in 1974–75 as a serious policy option. The relevant cabinet notes remained secret until 2005.