In the first half of the 20th century, Northern Ireland was largely spared the strife of the Civil War, but there were sporadic episodes of inter-communal violence between nationalists and unionists during the decades that followed partition. Although the Irish Free State was neutral during World War II, Northern Ireland as part of the United Kingdom was not, and became involved in the British war effort (albeit without military conscription as it was introduced in Great Britain). Belfast suffered a bombing raid from the German Luftwaffe in 1941.

In elections to the 1921–1972 regional government, the Protestant and Catholic communities in Northern Ireland each voted largely along sectarian lines, meaning that the Government of Northern Ireland (elected by "first past the post" from 1929) was controlled by the Ulster Unionist Party. Over time, the minority Catholic community felt increasingly alienated by the regional government, with further disaffection fuelled by practices such as gerrymandering of the local council in Derry, and alleged discrimination against Catholics in housing and employment which was also alleged to have taken place against Protestants

In August 1969, the regional government requested that the British Army be deployed to aid the police, who were exhausted after several nights of serious rioting. In 1970, the paramilitary Provisional IRA, which favoured the creation of a united Ireland, was formed and began a campaign against what it called the "British occupation of the six counties". Other groups, on both the unionist side and the nationalist side, participated in the violence and the period known as "The Troubles" began, resulting in over 3,600 deaths over the subsequent three decades. Owing to the civil unrest during "The Troubles", the British government suspended home rule in 1972 and imposed "direct rule" from Westminster.

There were several (ultimately unsuccessful) political attempts to end "The Troubles", such as the Sunningdale Agreement of 1973 and the Anglo-Irish Agreement of 1985. In 1998, following a Provisional IRA ceasefire and multi-party talks, the Belfast Agreement was concluded and ratified by referendum. This agreement attempted to restore self-government to Northern Ireland on the basis of power-sharing between the two communities. Violence decreased greatly after the signing of the accord, and on 28 July 2005, the Provisional IRA announced the end of its armed campaign and international weapons inspectors supervised what they currently regard as the full decommissioning of the Provisional IRA's weapons. The power-sharing assembly was suspended several times but restored from 8 May 2007.