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.
Irish issue in British politics
The Anglo-Irish War was fought between the forces of the provisional government of the Irish Republic and Britain, between January 1919 and July 1921. The British tried repressive force but the IRA under Michael Collins used about 3,000 fighters in asymmetric warfare. Prime Minister David Lloyd George presided over the Government of Ireland Act 1920 which partitioned Ireland into Southern Ireland and Northern Ireland in May 1921 during the Anglo-Irish War. Lloyd George used the threat of a massive war to force the Collins and his Irish negotiators to agree to the peace treaty in December 1922. It created a largely autonomous dominion, the Irish Free State, to replace the (failed) Southern Ireland. Meanwhile Northern Ireland, or Ulster, (with home rule) remained part of the United Kingdom. The Dail majority supported Collins and the treaty, but De Valera, President of the Dail, rejected it. De Valera led a civil war in Ireland that failed. The treaty with Britain went into effect and Northern Ireland remained with the UK.