Bloody Sunday was a British atrocity, and the biggest single killing by state forces in the course of the conflict in Northern Ireland.
The murder of 14 civil rights protesters was part of a wholesale repression of Catholics by the Northern Irish government and the British state.
British troops were sent into Northern Ireland in August 1969 because the armed sectarian police force could no longer contain an effective insurrection in Derry, the second largest city in Northern Ireland.
People were fighting back against a system where access to jobs, housing and effective votes depended on whether you were Catholic or Protestant.
The Labour government had to act to prop up a Unionist government that ran Northern Ireland as a sectarian, one party state.
At that time the IRA scarcely existed. But the brutality of the British army turned that around.
In July 1970 the British imposed a curfew on the Lower Falls Catholic area of Belfast. It wasn’t until February 1971 that the IRA killed its first British soldier.
On 9 August 1971 the army swooped into Catholic areas at dawn, dragging off 346 men to be interned – detained without trial, often for years.
Few were IRA activists. No Loyalists were taken. Nine civilians were shot dead as rioting spread in response.
Then came Bloody Sunday. A march was organised in opposition to internment – and was deemed illegal.
It was scheduled to begin in the Creggan area of Derry and to weave through the Bogside before proceeding to Guildhall Square in the city centre.
It never got that far. The paratroops went into the Bogside and opened fire.