Republic of Ireland–United Kingdom border
Originally intended as an internal boundary within the United Kingdom of Great Britain and Ireland, the border was created in 1921 under the United Kingdom Parliament's Government of Ireland Act 1920. Prior to this, a separatist Irish parliament had been established in Dublin, which did not recognise the Government of Ireland Act, and was actively engaged in the Irish War of Independence. The Act was intended to deliver Home Rule in Ireland, with separate parliaments for Southern Ireland (which included three of the nine counties of Ulster) and Northern Ireland. Six of the thirty-two counties of Ireland were assigned to Northern Ireland, and the rest of Ireland comprising 26 counties to Southern Ireland.
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.