Operation Banner was the operational name for the British Armed Forces' operation in Northern Ireland from 1969 to 2007, as part of the Troubles. It was the longest continuous deployment in British military history. The British Army was initially deployed, at the request of the unionist government of Northern Ireland, in response to the August 1969 riots. Its role was to support the Royal Ulster Constabulary (RUC) and to assert the authority of the British government in Northern Ireland. This involved counter-insurgency and supporting the police in carrying out internal security duties such as guarding key points, mounting checkpoints and patrols, carrying out raids and searches, riot control and bomb disposal. More than 300,000 soldiers served in Operation Banner. At the peak of the operation in the 1970s, about 21,000 British troops were deployed, most of them from Great Britain. As part of the operation, a new locally-recruited regiment was also formed: the Ulster Defence Regiment (UDR).

The Provisional Irish Republican Army (IRA) waged a guerrilla campaign against the British military from 1970 to 1997. Catholics welcomed the troops when they first arrived, because they saw the RUC as sectarian, but Catholic hostility to the British military's deployment grew after incidents such as the Falls Curfew (1970), Operation Demetrius (1971) and Bloody Sunday (1972). In their efforts to defeat the IRA, there were incidents of collusion between British soldiers and Ulster loyalist paramilitaries. From the late 1970s the British government adopted a policy of "Ulsterisation", which meant giving a greater role to local forces: the UDR and RUC. After the Good Friday Agreement in 1998, the operation was gradually scaled down, most military facilities were removed and the vast majority of British troops were withdrawn.

According to the Ministry of Defence, 1,441 serving British military personnel died in Operation Banner; 722 of whom were killed in paramilitary attacks, and 719 of whom died as a result of other causes. It suffered its greatest loss of life in the Warrenpoint ambush of 1979.

The British Army was initially deployed, at the request of the unionist government of Northern Ireland, in response to the August 1969 riots. Its role was to support the Royal Ulster Constabulary (RUC) and to assert the authority of the British government in Northern Ireland. The main opposition to the British military's deployment came from the Provisional Irish Republican Army (IRA). It waged a guerrilla campaign against the British military from 1970 to 1997. Catholics welcomed the soldiers when they first arrived in August 1969, but Catholic hostility to the British military's deployment increased after incidents such as the Falls Curfew (1970), Operation Demetrius (1971), the Ballymurphy Massacre (1971) and Bloody Sunday (1972). An internal British Army document released in 2007 stated that, whilst it had failed to defeat the IRA, it had made it impossible for the IRA to win through violence, and reduced substantially the death toll in the last years of conflict.

From 1998, after the Good Friday Agreement, Operation Banner was gradually scaled down: patrols were suspended and several military barracks closed or dismantled, even before the start of the decommissioning of IRA armaments. The process of demilitarisation started in 1994, after the first IRA ceasefire. From the second IRA ceasefire in 1997 until the first act of decommissioning of weapons in 2001, almost 50% of the army bases were vacated or demolished along with surveillance sites and holding centres, while more than 100 cross-border roads were reopened.

Eventually in August 2005, it was announced that in response to the Provisional IRA declaration that its campaign was over, and in accordance with the Good Friday Agreement provisions, Operation Banner would end by 1 August 2007. From that date troops were to be based in Northern Ireland only for training purposes, and reduced in number to 5,000; responsibility for security was entirely transferred to the police. The Northern Ireland–resident battalions of the Royal Irish Regiment – which grew out of the Ulster Defence Regiment – were stood down on 1 September 2006. The operation officially ended at midnight on 31 July 2007, making it the longest continuous deployment in the British Army's history, lasting over 37 years.

While the withdrawal of troops was welcomed by nationalist political parties the Social Democratic and Labour Party and Sinn Féin, the unionist Democratic Unionist Party and Ulster Unionist Party opposed the decision, which they regarded as 'premature'. The main reasons behind their resistance were the continuing activity of republican dissident groups, the loss of security-related jobs for the Protestant community, and the perception of the British Army presence as an affirmation of the political union with Great Britain.

Adam Ingram, the Minister of State for the Armed Forces, has stated that assuming the maintenance of an enabling environment, British Army support to the PSNI after 31 July 2007 was reduced to a residual level, known as Operation Helvetic, providing specialised ordnance disposal and support to the PSNI in circumstances of extreme public disorder as described in Patten recommendations 59 and 66, should this be needed, thus ending the British Army's emergency operation in Northern Ireland.