The chain of command from a cabinet table to a crack unit of assault troops in action is being stripped bare, slowly but surely, in the clinical confines of a legal assembly that convened in Derry this week.

For those who have watched the opening phase of the historical autopsy being attempted by the Bloody Sunday inquiry, it has been a dizzying, but stimulating, experience.

A week, we have learned, is a long time, not just in politics but also in the workings of a tribunal. Although this is only the beginning of a prolonged forensic process, it has already identified some key entry points for the coming legal incisions.

What has emerged is just the outline of the complex web of responsibility, communication and control that links the military and political arms of government at a time of crisis.

The mechanisms are being exposed. The exact sequence in which they operated, who initiated them at each stage and, most crucially, why, remain to be investigated. It will be a tortuous and at times inevitably tedious exercise.

Counsel for the tribunal, Mr Christopher Clarke QC, has, by a rough estimate, spoken at least half-a-million words in the first four days. He has covered immense ground, inviting the tribunal and its audience to taste minute samples of the great and varied body of evidence which has been gathered in two years of preliminary investigations.

He rejected the option of making a short statement about the issues raised by the advance material and diving straight into direct oral evidence by witnesses. Instead, he chose the radically different course of reviewing and partially analysing the voluminous material already to hand.

As a result, it is possible to detect some shapes and patterns. A strong impression has been gained, for example, of the perilous climate that pervaded the streets of Derry 28 years ago. Soldiers and civilians alike lived their daily lives on a knife-edge of tension and sporadic violence, driven and influenced by political pressures from a range of sources.

A series of Northern Ireland and British cabinet and cabinet committee minutes, top-level military and political briefing papers and policy drafts indicate the powerful external forces that were being brought to bear on events on the ground.

As 1971 drew to a close, there had been fatalities among both soldiers and citizens, the city's commercial life was being devastated by bombs and rioting, and the continued existence of so-called no-go areas in Bogside and Creggan - containing some 33,000 people - was an affront and a running sore to military commanders and to the political establishments in Stormont and London.

The inquiry has already heard evidence of the erosion this was causing to previously accepted codes of conduct and discipline among army units. Line commanders were increasingly frustrated and individual incidents of soldiers opening fire in questionable circumstances were multiplying.

High-ranking officers, including the Commander of Land Forces in Northern Ireland, Gen Robert Ford, had already drafted and submitted policy papers suggesting that new and drastic measures were needed, including even the shooting of ringleaders among the rioting youths.

In a briefing paper for the British prime minister, Mr Edward Heath, in October, the chief of the general staff of the army, Gen Sir Michael Carver, proposed "much more severe tactics", including: "Less restrictions on the use of non-lethal devices . . . and firearms, to permit tougher action with a view to deterring riots and hooliganism."

Into this atmosphere, in January 1972, as a major civil rights march to protest against internment loomed, somebody (and Gen Ford has so far taken sole responsibility) decided to introduce a battalion of the Parachute Regiment.

We have yet to hear an exploration of the training they received in the run-up to their arrival in Derry, a city where they had no experience on the ground and no appreciation of the community. They had already gained, and apparently taken pride in, a fearsome reputation for toughness and brutality in Belfast and elsewhere.

Why it was further decided to deploy them, rather than locally resident units and officers, into the Bogside at a time when thousands of "reasonable citizens" (as an officer of another unit put it) were gathering for a meeting, remains to be teased out.

Remarkably, the late Lord Widgery, in his now discredited report on Bloody Sunday, may have left a vital clue for the inquiry. The final decision on how the civil rights march was to be handled, he wrote, was taken "by higher authority" after Gen Ford and the chief constable had been consulted.

In the months to come, that line may bring the inquiry back again and again to the door of the cabinet room, as it seeks to establish the fine detail of the operational planning, and the chain of decision-making at every level, for Operation Forecast.