The Irish Civil War (June, 1922 - April, 1923) was a conflict between supporters and opponents of the Anglo-Irish Treaty of 6 December 1921 which had established the Irish Free State, precursor of today's Republic of Ireland.
Upon the Treaty's ratification (January 1922) by a narrow majority in the Dáil Éireann, the parliament established by the Sinn Féin MPs elected to Ireland's seats in the British parliament, Eamon de Valera resigned as President of the Republic, leading his anti-Treaty wing of Sinn Féin out of Dáil Éireann. He challenged the right of the Dáil to approve the Treaty, saying that its members were breaking their oath to the Irish Republic and attempted unsuccessfully to set up his own rival government.
The occupation (April 1922) by anti-Treaty militants of the Four Courts, the centre of judicial administration in Ireland, resulted in a tense stand-off which was only ended at British insistence by the building's bombardment and capture by Free State forces (June 28-June 30). In withdrawing, the Republicans boobytrapped the Irish Public Records Office, which was located next door. The result of this was the destruction of one thousand years of Irish state and religious archives. Pitched battles continued in Dublin until July 5.
With Dublin in Free State hands, conflict spread through the country, with anti-Treaty forces briefly holding Cork, Limerick and Waterford. Government victories in the major towns inaugurated a period of inconclusive guerrilla warfare marked by assassinations and executions of leaders formerly allied in the cause of Irish independence. The head of the Provisional Government, General Michael Collins was assassinated by Anti-Treaty republicans in August.
As the conflict petered out into a de facto victory for the pro-Treaty side, de Valera ordered a ceasefire, followed (May 1923) by an order to Republicans to dump their arms rather than surrender them or continue a fight which they were visibly incapable of winning for the time being.
As with most civil wars the internecine conflict left a bitter legacy, which continues to influence Irish politics to this day. The two largest political parties in the Republic are still Fianna Fáil and Fine Gael, the descendants respectively of the anti-Treaty and pro-Treaty forces of 1922.