due to an anticipated rise in "Nationalist/Republican" paramilitary activity as a result of "celebrations" associated with the 50th anniversary of the Easter Week Rising.
"On 7 May 1966, a group of UVF men led by Spence petrol bombed a Catholic-owned pub on the Shankill Road. Fire also engulfed the house next door, killing the elderly Protestant widow, Matilda Gould (77), who lived there.[18] On 27 May, Spence ordered four UVF men to kill an Irish Republican Army (IRA) member, Leo Martin, who lived on the Falls Road. Unable to find their target, the men drove around in search of any Catholic instead. They shot dead John Scullion (28), a Catholic civilian, as he walked home.[19] Spence later wrote "at the time, the attitude was that if you couldn't get an IRA man you should shoot a Taig, he's your last resort".[19] On 26 June, the same gang shot dead Catholic civilian Peter Ward (18) and wounded two others as they left a pub on Malvern Street, Belfast.[18] Two days later, the government of Northern Ireland declared the UVF illegal.[18] Shortly after, Spence and three others were arrested.[20]
In October 1966, Spence was sentenced to life imprisonment for the murder of Peter Ward, although Spence has always claimed he was innocent.[2] He was sent to Crumlin Road Prison. During its 12 July 1967 march, the Orange lodge to which he belonged stopped outside the prison in tribute to him.[21] This occurred despite Spence having been officially expelled from the Orange Order following his conviction.[22] Spence's involvement in the killings gave him legendary status among many young loyalists and he was claimed as an inspiration by the likes of Michael Stone.[23] Tim Pat Coogan has described Spence as a "loyalist folk hero".[24] The attack was however repudiated by Ian Paisley and condemned in his Protestant Telegraph, sealing the earlier split between the two.[25]"

These murders were the first examples of bloodletting in the troubles

"Were the OIRA and PIRA present and armed in the Bogside that day? Yes they most certainly were."
No evidence has been produced that they were and there was no evidence of arms apart from those used to gun down protesters
This is now officially recognised as the massacre of innocent people, though the culprits have yet to be punished for those murders.
The cause of the Troubles dates back to 1922 and the near half century of prejudice and injustice that followed it.
The Republican violence was inspired by police-supported brutality by Loyalist in response to peaceful demonstrations.

"During 12–17 August 1969, Northern Ireland was rocked by intense political and sectarian rioting. There had been sporadic violence throughout the year arising from the civil rights campaign, which was demanding an end to discrimination against Irish Catholics. Civil rights marches were repeatedly attacked by both Ulster Protestant loyalists and by the Royal Ulster Constabulary (RUC), a unionist and largely Protestant police force.
The disorder led to the Battle of the Bogside in Derry, a three-day riot in the Bogside district between the RUC and the nationalist/Catholic residents. In support of the Bogsiders, nationalists and Catholics launched protests elsewhere in Northern Ireland. Some of these led to attacks by loyalists working alongside the police. The most bloody rioting was in Belfast, where seven people were killed and hundreds more wounded. Scores of houses, most of them owned by Catholics, as well as businesses and factories were burned-out. In addition, thousands of mostly Catholic families were driven from their homes. In certain areas, the RUC helped the loyalists and failed to protect Catholic areas. Events in Belfast have been viewed by some as a pogrom against the Catholic and nationalist minority.[1][2][3]"
Jim Carroll