With the inquiry set to resume in Derry next week, Niall Ó Dochartaigh argues that the secret role of the city's police chief is key to understanding the context of the events.
The Widgery tribunal into the events of Bloody Sunday in 1972 steered sharply away from the background to the killings, focusing attention instead on the actions of individual soldiers and diverting attention from the decisions of senior military and political commanders.
As Christopher Clarke, chief counsel for the Bloody Sunday Inquiry, delivers his closing summation next week and Lord Saville prepares his completed report, they will be judged in large part on how they deal with the background to the killings and explain the context for the actions of soldiers on the ground.
Ultimately the inquiry will be required to adjudicate between two directly conflicting policy approaches in the security forces at the time. Understanding the role of Chief Supt Frank Lagan, RUC commander in Derry at the time, is central to this task.
Those who defend the actions of the soldiers stress the context of intense IRA activity in the months before the march, the sense of threat and the radically unpredictable nature of the situation the soldiers faced.
Yet Chief Supt Lagan testified in his statement to the inquiry that he believed there would not be shooting by the IRA during the march.
And one key witness, Brendan Duddy, a businessman who had been active in the civil rights campaign in Derry and had an extensive network of political contacts, worked closely with Lagan in the months before Bloody Sunday and has testified credibly that Lagan asked him to seek assurances from the Official and Provisional IRA that there would be no guns on the march.
While Lagan made no reference to this in his statement, this evidence places him at the centre of the debate on whether the army was expecting the IRA to pose a major threat during the march.
Lagan was also deeply involved in trying to influence army planning for the march. He initially urged the army to allow the march into the city centre, later conveyed the news to army commanders that the march organisers had decided not to try to breach army barriers and finally urged the local army commander not to allow the Paras to go into the Bogside in those fateful minutes before they rushed forward.
Lagan's actions provide clear illustration that an alternative approach to the march was promoted within the security forces and that army action did not unfold as an inevitable response to the situation or to the threat posed by the IRA.
Lagan's role on the day can in turn only be understood in the context of three years of intermittent communication and negotiation between the security forces and a range of forces in the Catholic community in Derry as violence escalated in the approach to Bloody Sunday.
Lagan, himself a Catholic from Co Derry who had gone to school in Derry city, played a key role on a number of occasions in brokering agreements aimed at averting or reducing violence through reductions in security force activity.
These agreements were hugely problematic and flawed, but on a number of occasions they led to a direct avoidance of violence or reductions of violence around key events or at times of extreme tension. In these negotiations and communications Lagan drew on a huge range of contacts in the Catholic community, some of them shrouded in secrecy. In the approach to Bloody Sunday he made desperate attempts to avert confrontation once again.
Lagan's appointment to head the RUC in Derry was a key element in the reform of the force, aimed at restoring Catholic support for the police in Derry where Catholics had been comprehensively alienated from them. Lagan's approach fitted well with British government support for reform in 1969 and early 1970, but by late 1971 his approach was clearly at odds with a Stormont government policy of increasing repression, backed by a new Conservative government in Westminster.
After Bloody Sunday senior army officers represented Lagan as a figure the army could not rely on because of his close connections, and sympathy with, the Catholic community. But this is a false contradiction. Army commanders in Derry relied on Lagan, in the years and months before Bloody Sunday, precisely because of these connections.
Although Lagan was swimming against the tide by late 1971 he was by no means an isolated or marginal figure. He had huge influence on army commanders in Derry, and it seems that the strength of his influence was at the heart of Gen Robert Ford's discontent with security-force policy in Derry.
Ford, as Commander Land Forces, was a key driver of army policy in the North and personally devised Operation Forecast, the army plan for dealing with the march on Bloody Sunday.
The fury that Lagan's actions on and after Bloody Sunday aroused in some army and RUC circles is evident from several of the documents turned up by the inquiry. But in the wake of Bloody Sunday Lagan not only survived, he was promoted and remained in charge in Derry for another four years.
Convincing evidence has emerged from recently released public records and recent interviews with a key figure in Derry who maintained regular contact with Lagan in these years, that Lagan enjoyed the direct support and protection of senior British civil servants for much of his tenure in Derry, figures who supported and encouraged his approach.
When an intensely secret channel facilitating communication between the British government and the Provisional IRA was established in 1972 Chief Supt Lagan facilitated the practicalities of these communications, according to one of those centrally involved. This channel was kept secret and was used, on and off, for the following two decades.
It was through this that the communication and negotiation that culminated in the IRA ceasefire of 1994 began. It is possible to draw a direct line from the network of contacts Lagan was involved in in 1972, and which he utilised to try to avert confrontation on Bloody Sunday, and the channels through which an IRA ceasefire was finally brokered in the early 1990s.
Lagan was a key figure in a persistent, but by no means dominant, strand in British policy in the North, a strand that emphasised negotiation and communication, even in the face of an escalating IRA campaign.
Disagreement between Lagan and senior army commanders on the way to deal with the march are best understood, not as a disagreement on the most effective approach to the march, nor as a contest between important senior army officers and a marginal policeman, but as a confrontation between two opposed policy approaches, both of which were well established within the security forces.
At the heart of Operation Forecast was the intention to make a radical break with a local policy in Derry which had strong elements of conciliation and restraint.
The crucial context for Bloody Sunday is a clash between two directly opposing policy approaches within the security forces. Lord Saville's task will necessarily involve adjudicating between the two.
Niall O Dochartaigh is a lecturer in political science and sociology at the National University of Ireland, Galway. He is the author of From Civil Rights to Armalites: Derry and the Birth of the Irish Troubles. The second edition will be published by Palgrave Macmillan in December