The British responded to the potent Catholic nationalism with the Home Rule Act of 1914, but then suspended it until after the First World War. In Dublin, the rising tide of militant Irish nationalism was manifest in the Easter rising of 1916. The harsh treatment of leaders by the British alienated the Irish and led to further radicalisation of the nationalist party Sinn Fein under Arthur Griffith and Eamon de Valera, and the expansion of the Irish Republican Army (IRA) under Michael Collins. In April 1918 an attempt by the British government to introduce conscription produced mass demonstrations. By the end of the year resentment of British rule was widespread in Ireland.
In December 1918 Sinn Fein won 73 seats out of 105 Irish seats in the general election, eclipsing the Irish Party. Sinn Fein MPs met in Dublin in January 1919 and set up the Dail Eireann (Irish parliament). The Dail immediately reaffirmed the republic of 1916 and set up a government in opposition to the British administration at Dublin Castle.
On the same day as the reaffirmation of the republic on 19 January 1919, a small number of IRA members shot two Royal Irish Constabulary (RIC) members at Soloheadbeg in Tipperary. This incident began the Irish War of Independence (although the Dail did not declare war until April). Towards the end of 1919 Cabinet considered the Government of Ireland Bill, which was enacted in 1920 and provided for Ulster provinces to remain within the United Kingdom.
British policy towards the rebellion was indecisive at first, but by May 1920 the gravity of the situation became apparent. Irish ostracism of the Royal Irish Constabulary (RIC) was effective and strikes prevented the movement of troops. By mid-1920 British authority in Ireland was collapsing.
In May 1920 the Foreign Secretary, Lord Curzon, proposed vigorous 'Indian measures' to suppress the rebellion. The Restoration of Order in Ireland Act was passed in August 1920 and suspended the courts system, instead establishing military courts with powers to enforce the death penalty and internment without trial. As the RIC became increasingly ineffective the British government deployed the Auxiliary and Black and Tans divisions of the RIC, made up of British ex-servicemen who were essentially mercenaries, to suppress the rebellion.
Violence intensified in November 1920 when Auxiliaries shot into a crowd of sports spectators, killing 14 people. This provoked violence on the streets and is remembered as 'Bloody Sunday'. In May 1921 Sinn Fein won an overwhelming victory in the parliamentary election. The British government intended to rule Ireland as a crown colony and dissolved the parliament under the Government of Ireland Act. The Act caused another upsurge of violence - Britain's Irish policy had essentially failed.
Left with no other constructive policies, the British Cabinet agreed to talks with Sinn Fein. The truce came into force on 11 July 1921. The Anglo-Irish Treaty of December 1921 gave Ireland dominion status within the Empire and momentously gave Ulster the right to opt out of a unified Ireland and remain part of the United Kingdom.