One of the great unanswerable questions of The Troubles is whether the slide into large-scale violence at the end of the 1960s could have been avoided - was it inevitable?
With hindsight, few people who were there at the time doubt that the writing had been on the wall for some years - but many have struggled to identify the moment when the "what ifs" of history could have changed the course of Northern Ireland.
While the first actual killings of the Troubles came a few years earlier, the point at which civil order totally broke down was during the height of the civil rights protests of 1969.
On New Year's Day 1969 a small group of student radicals, the People's Democracy, set off from Belfast City Hall on a four-day, 75-mile protest march to Londonderry/Derry.
On its final day, loyalist opponents ambushed the students at Burntollet just outside Derry. The students claimed they were led into the ambush by the police, something that the police denied.
But some of those who attacked the students were later identified as B Specials, a controversial and since disbanded reserve unit of the RUC.
Rioting followed and barricades went up in nationalist areas of the city. The Bogside area of Derry was declared a "no-go area" for the authorities. The protesters - peaceful or otherwise - dubbed the area "Free Derry", a name that has stuck to this day.
As the Catholic community gained in confidence in the growing campaign for fair rights and equality, there appeared to be a reciprocal rise in tension among loyalists who saw the Protestant dominance of Northern Ireland under threat.
They were determined to press ahead with the summer marching season but trouble flared increasingly in flashpoint areas.
On 12 August, an Apprentice Boy parade in Derry led to clashes with local Catholic youths. Police, who tried to separate the factions, were stoned and petrol-bombed.
Police entered the Catholic Bogside area with armoured vehicles and water cannons. The rioting, which came to be known as the "Battle of the Bogside" lasted two days.
But more serious violence erupted in Belfast. The official report into what happened stated that "Prostestant mobs made a determined effort" to enter a Catholic area of the city. The police attempted to resist the violence but came under attack from loyalists. Republicans have since described it as an attempted pogrom against Catholics.
As the violence increased and Catholic homes in some areas of Belfast were set alight, the police believed that they were facing an "armed uprising" by loyalists in some areas and sprawling rioting by Catholics in others. The police were losing control.
On 14 August, Labour Home Secretary, Jim Callaghan, ordered troops on to the streets of Northern Ireland. He said their goal was to restore law and order. As they arrived in positions throughout Belfast and Derry, Catholic families welcomed them with open arms. Within 18 months all was to change.