Irish Civil War in the context of "Oath of Allegiance (Ireland)"

The 1921 Anglo-Irish Treaty (Irish: An Conradh Angla-Éireannach), commonly known in Ireland as The Treaty and officially the Articles of Agreement for a Treaty Between Great Britain and Ireland, was an agreement between the government of the United Kingdom of Great Britain and Ireland and the government of the Irish Republic that concluded the Irish War of Independence. It provided for the establishment of the Irish Free State within a year as a self-governing dominion within the "community of nations known as the British Empire", a status "the same as that of the Dominion of Canada". It also provided Northern Ireland, which had been created by the Government of Ireland Act 1920, an option to opt out of the Irish Free State (Article 12), which was exercised by the Parliament of Northern Ireland.

The agreement was signed in London on 6 December 1921, by representatives of the British government (which included Prime Minister David Lloyd George, who was head of the British delegates, and Winston Churchill, who was Secretary of State for the Colonies) and by representatives of the government of the Irish Republic (which included Michael Collins, who was Secretary of State for Finance, and Arthur Griffith, who was Secretary of State for Foreign Affairs). The Irish representatives had plenipotentiary status (negotiators empowered to sign a treaty without reference back to their superiors) acting on behalf of the Irish Republic, though the British government declined to recognise that status. As required by its terms, the agreement was approved by "a meeting" of the members elected to sit in the House of Commons of Southern Ireland and [separately] by the British Parliament. In reality, Dáil Éireann (the legislative assembly for the de facto Irish Republic) first debated then approved the treaty; members then went ahead with the "meeting". Though the treaty was narrowly approved, the split led to the Irish Civil War, which was won by the pro-treaty side.

The Irish Republican Army (IRA; Irish: Óglaigh na hÉireann) was an Irish republican revolutionary paramilitary organisation who waged a guerrilla campaign against the British occupation of Ireland in the 1919–1921 Irish War of Independence. It was descended from the Irish Volunteers, an organisation established on 25 November 1913 that staged the Easter Rising in April 1916. In 1919, the Irish Republic that had been proclaimed during the Easter Rising was formally established by an elected assembly (Dáil Éireann), and the Irish Volunteers were recognised by Dáil Éireann as its legitimate army.

Following the signing in 1921 of the Anglo-Irish Treaty, which ended the War of Independence, a split occurred within the IRA. Members who supported the treaty formed the nucleus of the Irish National Army. However, the majority of the IRA was opposed to the treaty. The anti-treaty IRA fought a civil war against the Free State Army in 1922–1923, with the intention of creating a fully independent all-Ireland republic. Having lost the civil war, this group remained in existence, with the intention of overthrowing the governments of both the Irish Free State and Northern Ireland and achieving the Irish Republic proclaimed in 1916.

Michael Collins (Irish: Mícheál Ó Coileáin; 16 October 1890 – 22 August 1922) was an Irish revolutionary, soldier and politician who was a leading figure in the early-20th century struggle for Irish independence. During the War of Independence he was Director of Intelligence of the Irish Republican Army (IRA). He served in the government of the self-declared Irish Republic as the Minister for Home Affairs and later as the Minister for Finance. He was Chairman of the Provisional Government of the Irish Free State from January 1922 and commander-in-chief of the National Army from July until his death in an ambush in August 1922, during the Civil War.

Collins was born in Woodfield, County Cork, the youngest of eight children. He moved to London in 1906 to become a clerk in the Post Office Savings Bank at Blythe House. He was a member of the London GAA, through which he became associated with the Irish Republican Brotherhood and the Gaelic League. He returned to Ireland in January 1916 and fought in the Easter Rising. He was taken prisoner and held in the Frongoch internment camp as a prisoner of war, but he was released in December 1916.

Irish republicanism (Irish: poblachtánachas Éireannach) is the political movement for an Irish republic, void of any British rule. Throughout its centuries of existence, it has encompassed various tactics and identities, simultaneously elective and militant and has been both widely supported and iconoclastic.

The modern emergence of nationalism, democracy, and radicalism provided a basis for the movement, with groups forming across the island in hopes of independence. Parliamentary defeats provoked uprisings and armed campaigns, quashed by British forces. The Easter Rising, an attempted coup that took place in the midst of the First World War, provided popular support for the movement. An Irish republic was declared in 1916 and officialized following the Irish War of Independence. The Irish Civil War, beginning in 1922 and spurred by the partition of the island, then occurred.

Sinn Féin (/ʃɪn ˈfeɪn/ shin FAYN; Irish: [ˌʃɪn̠ʲ ˈfʲeːnʲ] ; lit. '[We] Ourselves') is an Irish republican and democratic socialist political party active in both the Republic of Ireland and Northern Ireland.

The original Sinn Féin organisation was founded in 1905 by Arthur Griffith. Its members founded the revolutionary Irish Republic and its parliament, the First Dáil, and many of them were active in the Irish War of Independence, during which the party was associated with the Irish Republican Army (1919–1922). The party split before the Irish Civil War and again in its aftermath, giving rise to the two traditionally dominant parties of Irish politics: Fianna Fáil, and Cumann na nGaedheal (which merged with smaller groups to form Fine Gael). For several decades the remaining Sinn Féin organisation was small and often without parliamentary representation. It continued its association with the Irish Republican Army. Another split in 1970 at the start of the Troubles led to the modern Sinn Féin party, with the other faction eventually becoming the Workers' Party.

The revolutionary period in Irish history was the period in the 1910s and early 1920s when Irish nationalist opinion shifted from the Home Rule-supporting Irish Parliamentary Party to the republican Sinn Féin movement. There were several waves of civil unrest linked to Ulster loyalism, trade unionism, and physical force republicanism, leading to the Irish War of Independence, the Partition of Ireland, the creation of the Irish Free State, and the Irish Civil War.

Some modern historians define the revolutionary period as the period from the introduction of the Third Home Rule Bill to the end of the Civil War (1912/1913 to 1923), or sometimes more narrowly as the period from the Easter Rising to the end of the War of Independence or the Civil War (1916 to 1921/1923).