The Arnon Street Massacre takes place on April 1, 1922, in Belfast, Northern Ireland. Six Catholic civilians, three in Arnon Street, are shot dead. It is believed that members of either the Ulster Special Constabulary (USC) or the Royal Irish Constabulary (RIC) are responsible, acting in retaliation for the killing of an RIC officer by the Irish Republican Army (IRA).

Although the Irish War of Independence officially ends in July 1921, the Irish Republican Army’s conflict with British and unionist forces continues in Northern Ireland and escalates in the first half of 1922. The Ulster IRA, with the tacit but covert assistance of Michael Collins, head of the new Irish Free State, continues to wage a guerrilla war in Northern Ireland. According to historian Alan Parkinson, despite “the IRA having some short-term successes … the main effect of this intensive campaign was to unleash a terrible backlash on the Catholic population in Belfast.” Only a week before the Arnon Street incident, policemen – either Royal Irish Constabulary (RIC) or Ulster Special Constabulary (USC) – kill six Catholic civilians in the McMahon murders.

On the evening of April 1, RIC constable George Turner is patrolling the Old Lodge Road when he is killed by a sniper.

About ten police officers in Brown Square Barracks, upon hearing of Turner’s murder, take a Lancia armoured car and begin touring nationalist areas. When they dismount their vehicle, witnesses hear them shouting “Cut the guts out of them for the murder of Turner.” Their first victim is John McRory who lives on Stanhope Street, just across the road from where Constable Turner had been shot. The police break into his house and shoot him dead in his kitchen. In Park Street, Bernard McKenna, father of seven, is killed while lying in bed. Finally, the police arrived at Arnon Street.

William Spallen, who lives at 16 Arnon Street, has just returned from the funeral of his wife who had also been killed in the conflict. His 12-year-old grandson, Gerald Tumelty, witnesses his death. “Two men came into the room, one was in the uniform of a policeman. They asked my grandfather his name and he said William Spallen. The man in plain clothes fired three shots at him. When I cried out, he said, ‘lie down or I will put a bullet into you.'” Tumelty says the killers then take £20 that his grandfather had to pay for his wife’s funeral.

The attackers then use a sledgehammer to break into the house next door, where they find Joseph Walsh in bed with his seven-year-old son Michael and his two-year-old daughter Bridget. Joseph Walsh is bludgeoned to death with the sledgehammer while Michael Walsh is shot and dies from his wounds the next day. Another son, Frank, is shot in the thigh but survives. Later that evening another Catholic, John Mallon, is shot dead in Skegoneill Avenue.

The unionist press, the Belfast News Letter and Belfast Telegraph, condemn the killings but do not identify the killers as police. The Dublin-based Irish Independent writes that “never even in the worst state of terror in the west and south has the state of affairs which now prevails in the Northern capital been experienced.” Michael Collins sends an angry telegram to Northern Ireland Prime Minister James Craig, demanding a joint inquiry into the killings. No such inquiry is set up.

As with the McMahon killings one week earlier, it is strongly suspected that an RIC Detective Inspector, Nixon, operating out of the Brown Street Police barracks, had organised the attack. Nixon and several other policemen fail to turn up at roll call at the barracks immediately after the killings.