Operation Demetrius was a British Army operation in Northern Ireland on 910 August 1971, during the Troubles. It involved the mass arrest and internment (imprisonment without trial) of people suspected of being involved with the Irish Republican Army (IRA), which was waging an armed campaign for a united Ireland against the British state. It was proposed by the Unionist government of Northern Ireland and approved by the British Government. Armed soldiers launched dawn raids throughout Northern Ireland and arrested 342 in the initial sweep, sparking four days of violence in which 20 civilians, two IRA members and two British soldiers were killed. All of those arrested were Irish republicans and nationalists, the vast majority of them Catholics. Due to faulty and out-of-date intelligence, many were no longer involved in republican militancy or never had links with the IRA. Ulster loyalist paramilitaries were also carrying out acts of violence, which were mainly directed against Catholics and Irish nationalists, but no loyalists were included in the sweep.The introduction of internment, the way the arrests were carried out, and the abuse of those arrested, led to mass protests and a sharp increase in violence. Amid the violence, about 7,000 people fled or were forced out of their homes.

The policy of internment lasted until December 1975 and during that time 1,981 people were interned; 1,874 were nationalist, while 107 were loyalist. The first loyalist internees were detained in February 1973.The interrogation techniques used on some of the internees were described by the European Commission of Human Rights in 1976 as torture, but the superior court, the European Court of Human Rights (ECHR), ruled on appeal in 1978 that while the techniques were "inhuman and degrading", they did not constitute torture in this instance. It was later revealed that the British government had withheld information from the ECHR and that the policy had been authorized by British government ministers. In light of the new evidence, in 2014 the Irish government asked the ECHR to revise its judgement, but the ECHR eventually declined the request.

The Troubles (Irish: Na Trioblóidí) were an ethno-nationalist conflict in Northern Ireland that lasted about 30 years from the late 1960s to 1998. Also known internationally as the Northern Ireland conflict, it is sometimes described as an "irregular war" or "low-level war". The conflict began in the late 1960s and is usually deemed to have ended with the Good Friday Agreement of 1998. Although the Troubles mostly took place in Northern Ireland, at times violence spilled over into parts of the Republic of Ireland, England and mainland Europe.

The conflict was primarily political and nationalistic, fuelled by historical events. It also had an ethnic or sectarian dimension but despite use of the terms 'Protestant' and 'Catholic' to refer to the two sides, it was not a religious conflict. A key issue was the status of Northern Ireland. Unionists and loyalists, who for historical reasons were mostly Ulster Protestants, wanted Northern Ireland to remain within the United Kingdom. Irish nationalists and republicans, who were mostly Irish Catholics, wanted Northern Ireland to leave the United Kingdom and join a united Ireland.

The conflict began during a campaign by the Northern Ireland Civil Rights Association to end discrimination against the Catholic/nationalist minority by the Protestant/unionist government and local authorities. The government attempted to suppress the protests. The police, the Royal Ulster Constabulary (RUC), were overwhelmingly Protestant and accused of sectarianism and police brutality. The campaign was also violently opposed by loyalists, who said it was a republican front. Increasing tensions led to the August 1969 riots and the deployment of British troops, in what became the British Army's longest operation. "Peace walls" were built in some areas to keep the two communities apart. Some Catholics initially welcomed the British Army as a more neutral force than the RUC, but soon came to see it as hostile and biased, particularly after Bloody Sunday in 1972.The main participants in the Troubles were republican paramilitaries such as the Provisional Irish Republican Army (IRA) and the Irish National Liberation Army (INLA); loyalist paramilitaries such as the Ulster Volunteer Force (UVF) and Ulster Defence Association (UDA); British state security forces such as the British Army and RUC; and political activists. The security forces of the Republic of Ireland played a smaller role. Republicans carried out a guerrilla campaign against British forces as well as a bombing campaign against infrastructural, commercial and political targets. Loyalists attacked republicans/nationalists and the wider Catholic community in what they described as retaliation. At times, there were bouts of sectarian tit-for-tat violence, as well as feuds within and between paramilitary groups. The British security forces undertook policing and counter-insurgency, primarily against republicans. There were incidents of collusion between British state forces and loyalist paramilitaries. The Troubles also involved numerous riots, mass protests and acts of civil disobedience, and led to increased segregation and the creation of temporary no-go areas.

More than 3,500 people were killed in the conflict, of whom 52% were civilians, 32% were members of the British security forces and 16% were members of paramilitary groups. Republican paramilitaries were responsible for some 60% of the deaths, loyalists 30% and security forces 10%. The Northern Ireland peace process led to paramilitary ceasefires and talks between the main political parties, which resulted in the Good Friday Agreement of 1998. This Agreement restored self-government to Northern Ireland on the basis of "power-sharing" and it included acceptance of the principle of consent, commitment to civil and political rights, parity of esteem, police reform, paramilitary disarmament and early release of paramilitary prisoners. There has been sporadic violence since the Agreement, including punishment attacks and a campaign by dissident republicans.