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–United States relations
The Irish War of Independence ultimately ended with the Anglo-Irish Treaty, which confirmed the partition of Ireland into the Irish Free State and Northern Ireland, the latter of which opted to remain a part of the United Kingdom. The Irish Free State quickly fell into the Irish Civil War between Pro-Treaty Forces who supported independence via partition and Anti-Treaty Forces who opposed partition and wanted independence for the entire island of Ireland. Pro-Treaty Forces won the Irish Civil War in 1923, and the following year the United States recognized the Irish Free State and established diplomatic relations with it. The Irish Free State was succeeded by the new state of Ireland in 1937, and formally declared itself a republic in 1949.