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 Civil War
The fact that the Irish Civil War was fought between Irish Nationalist factions meant that the sporadic conflict in Northern Ireland ended. Collins and Sir James Craig signed an agreement to end it on 30 March 1922, but, despite this, Collins covertly supplied arms to the Northern IRA until a week before his death in August 1922. Because of the Irish Civil War, Northern Ireland was able to consolidate its existence and the partition of Ireland was confirmed for the foreseeable future. The continuing war also confirmed the northern Unionists' existing prejudices against the ethos of all shades of nationalism. This might have led to open hostilities between North and South had the Irish Civil War not broken out. Indeed, the Ulster Special Constabulary (the "B-Specials") that had been established in 1920 (on the foundation of Northern Ireland) was expanded in 1922 rather than being demobilised.