During World War I, tensions continued to mount in Ireland. Hardline Irish separatists (known at the time as Irish Irelanders and later as Republicans) rejected Home Rule entirely because it involved maintaining the connection with Britain. They retained control of one faction of the Irish Volunteers, and in Easter, 1916 led by Thomas Clarke and James Connolly and others attempted a rebellion in Dublin. After summary trials, the British government had the leaders executed for treason. The government blamed the small Sinn Féin party, which had had little to do with it. The execution of the leaders of the rebellion turned out to be a propaganda coup for militant republicanism, and Sinn Féin's previously negligible popular support grew. The surviving leaders of the Irish Volunteers infiltrated the party and assumed leadership in 1917. (The Irish Volunteers themselves would later become the Irish Republican Army in 1919.)
Republicans gained further support when the British government attempted to introduce conscription to Ireland in 1918. Sinn Féin was at the forefront of organising the campaign against conscription.
When the veterans of World War I, on both sides of the political divide, returned from the front in 1918 and 1919, they came back as battle-hardened soldiers rather than the rag-tag yeomanry they had emerged from at the start of the war. In the general election of 1918, The Irish Parliamentary Party lost almost all of its seats to Sinn Féin. Unionists won 23 of 30 seats in the future Northern Ireland, and five of the six IPP members returned in Ireland were elected in Ulster as a result of local voting pacts with Sinn Féin.[7]
Guerrilla warfare raged across Ireland in the aftermath of the election, leading to the Anglo-Irish War. Although lower in intensity in the north than in the south, the conflict was complicated by involving not only the Irish Republican Army (IRA) and the British Army and Royal Irish Constabulary but the Ulster Volunteer Force (UVF) as well.
