In 1969 British troops were sent to Derry and Belfast to maintain order and to protect the Catholic minority. However, the army soon came to be seen as a tool of the Protestant majority by the minority Catholic community. This was reinforced by events such as Bloody Sunday in 1972 when British forces opened fire on a Catholic civil rights march in Derry killing 13 people. An escalation of paramilitary violence followed with many atrocities committed by both sides. The period of ‘the Troubles’ are generally agreed to have finished with the Belfast (or Good Friday) Agreement of April 10th 1998.
The war left the Irish nationalist parties highly polarised and embittered. The total casualty list has still not been definitively determined but appears to be about 1,500-2,000 killed with some thousands more injured. The anti-Treatyites entered politics as Fianna Fail in 1927 and came to power peacefully in 1932 – despite widespread rioting between the IRA and the pro-Treaty Blueshirt movement . By 1939, most of what they considered the objectionable features of the Treaty had been removed by acts of parliament. They and Fine Gael (pro-Treaty) dominated Irish politics for most of the 20th century.
However, the last major spasm of violence was in Northern Ireland, whose existence was confirmed under the Treaty. In early 1922, both pro and anti-Treaty wings of the IRA fought a clandestine campaign against Northern Ireland, tacitly supported by elements of the Provisional Free State Government led by Michael Collins. This culminated in a failed IRA offensive in May 1922, in which the guerrillas fought a number of sizable engagements with British troops at the villages of Pettigo and Beleek in Fermanagh, but overall failed to coordinate their actions and were imprisoned in large numbers by the Northern government.