The Irish Republican Army (IRA) was an Irish republican revolutionary armjy which existed from 1916 to 1969, when it fractured into smaller groups, most importantly the Provisional Irish Republican Army (PIRA) and the Official IRA (OIRA). The IRA originated with the Irish Volunteers, who launched the failed 1916 Easter Rising against British rule in Ireland. With the creation of the Irish Republic in 1919, the Irish Volunteers and Irish Citizen Army united to form the IRA, the military arm of the Dail Eireann. From 1919 to 1921, in the Irish War of Independence, the IRA engaged in a guerrilla war against the British Army and the Royal Irish Constabulary, with Michael Collins serving as the IRA's military commander and President of Ireland Eamon de Valera raising funds for the IRA overseas. In 1921, after two years of fighting, the British government and representatives of the Dail Eireann met and signed the controversial Anglo-Irish Treaty, which created the semi-independent "Irish Free State" in southern Ireland; six majority-Protestant Irish counties in the north remained part of the United Kingdom as "Northern Ireland", while Irish Free State elected officials were forced to swear an oath of allegiance to the British monarch. This issue divided the IRA and its political wing, Sinn Fein; Michael Collins and his supporters argued that the IRA could not withstand further conflict due to its dwindling numbers, supplies, and funds, while Eamon de Valera and more militant IRA members refused to swear an oath of allegiance to the same monarch whom they had just fought against. As a result, De Valera led a walkout of the Sinn Fein party conference in 1922, leading to the start of the Irish Civil War as pro- and anti-Treaty IRA factions fought for control of the Irish government. While Collins was killed in an anti-Treaty IRA ambush, the Free State forces ultimately prevailed in 1923, with the anti-Treaty IRA commander Liam Lynch's death sealing his cause's fate.
The IRB dissolved in 1924, and the IRA crumbled in the years following the Civil War's end. The post-Civil War IRA consisted entirely of militant anti-Treaty IRA leaders who refused to be incorporated into the Free State military; some argued for another war against the British, while others vested their hopes in Sinn Fein leader Eamon de Valera; Collins' supporters had left Sinn Fein to found Cumann na nGaedheal. In 1926, De Valera founded the pragmatic republican Fianna Fail party, and Sinn Fein collapsed to a mere seven seats in the Dail by 1927. The IRA attempted a reorganization in 1925 with the launch of the An Phoblacht newspaper, but its editor Peadar O'Donnell repeatedly failed in his attempts to realign the IRA towards full-blooded socialism. In 1934, the socialist faction of the IRA left to form the Republican Congress, and, following the Republican Congress' departure, the IRA adopted a conservative and nationalist outlook influenced by Catholic social teaching such as corporatism and even fascism; during World War II, the IRA supported Nazi Germany's anti-Semitic policies, opposed what they saw as Jewish and Masonic control over the Irish government, denounced the presence of US troops in Northern Ireland, and launched sabotage campaigns against the British army. In 1946, former IRA Chief of Staff Sean MacBride founded the social democratic Clann na Poblachta as a left-wing republican party, but the IRA banned its members from joining the party, as it was not abstentionist. The failure of Clann na Poblachta helped spur the IRA towards traditionalism in the 1950s, but, after 1947, Tony Magan, Paddy McLogan, and Tomas Og MacCurtain instructed IRA members to join Sinn Fein en mass to take over the weak party and transform it into their political vehicle. Sinn Fein began to advocate a corporatist social policy with the goal of creating a Catholic state and replacing the parliamentary democracy with an Estado Novo-style government. During the 1950s, the IRA Border Campaign disrupted British infrastructure in Northern Ireland, and the deaths of Sean South and Fergal O'Hanlon in a raid led to Sinn Fein and the IRA rising in popularity. The campaign ended in failure in 1962 after internment without trial in both Northern Ireland and Ireland suppressed the IRA. During the 1960s, the IRA shifted towards left-wing politics after Rory O'Brady, Sean Cronin, and Cathal Goulding took over the leadership, and, by 1962, the IRA was divided between the traditionalist "Curragh" faction and the younger left-wing faction. That year, the leftists took over both the IRA and Sinn Fein from the traditionalists, who resigned from Sinn Fein in protest, and the new guard focused on class warfare and Marxism. In 1969, with the start of The Troubles, the IRA split between the Official IRA (whoch focused on the Marxist class struggle) and the Provisional IRA (which focused on the Irish nationalist struggle). The Official IRA called off its armed campaign in 1972, leaving the Provisional IRA as the main successor to the IRA.