The complete break down of civil order in Northern Ireland during the summer of 1969 caused the British government to commit the British Army to keeping the rioting factions apart. The arrival of British troops was welcomed by the Catholics in Northern Ireland who saw the soldiers as peacekeepers - troops who would protect them from Protestant mobs. Unfortunately, the targeting of the army by the IRA, and measures taken to try to stop provisional IRA attacks, led to the alienation of the Catholics from the soldiers who were trying to protect them by the late spring 1970.
Internment - detention without trial - was introduced on 9 August 1971 and there was a massive surge in sectarian violence. Unfortunately for the security services, high-quality intelligence about who was or was not involved in terrorism - needed to make internment a success - was not available. Coupled to a lack of political support from the nationalist community, it meant that this time, unlike during the IRA's cross-border campaign, internment was ineffective and merely heightened divisions within Ulster. Internment was finally abandoned in 1975.
Internment was extremely unpopular and saw a sharp rise in the level of violence on both sides of the sectarian divide. It also saw a concerted peaceful campaign against the policy. On Sunday 30 January 1972 about 10,000 people attended a rally and march in Londonderry organised by the Northern Ireland Civil Rights Association. The Ulster parliament at Stormont had declared such marches illegal and the army had been ordered to prevent the march reaching the Guildhall.
What actually happened on the day has not been fully established. Soldiers from the Parachute Regiment claim they came under fire from gunmen within the march and from the nearby Rossville flats. The marchers say there were no terrorists, and that soldiers shot unarmed civilians without warning. The facts are that 13 people were shot dead, one died some time later of his injuries and many more were wounded. With the violence seemingly out of control and the ability of the Northern Ireland government to control its response in doubt, the Northern Ireland parliament at Stormont was suspended in March 1972 and direct rule from Westminster imposed.