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.
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.