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.
Dissident Irish Republican campaign
Since the 1169 invasion of Ireland by Norman knights at the request of ousted King of Leinster Dermot MacMurrough, Ireland has, in part or in whole, been under English, and later British, administration. Rebellions against rule from Great Britain were unsuccessful until 1919–1921's Anglo-Irish War, when the original Irish Republican Army (IRA) succeeded in removing 26 of Ireland's 32 counties from the United Kingdom of Great Britain and Ireland as the Irish Free State. Although previously, the offer of Irish Home rule was agreed in the third Home Rule Bill, implementation was suspended by violent opposition in Ulster and the forming of the UVF, and later by the outbreak of the First World War. Following the partition of Ireland by the Crown, the remaining six counties, located in the province of Ulster, became Northern Ireland and remained a part of the renamed United Kingdom of Great Britain and Northern Ireland.