The declassified documents show that the General Officer Commanding, Lieutenant General Sir Harry Tuzo, enlisted the UDA as a sort of fifth column. He let them act as auxiliaries to patrols by the Royal Ulster Constabulary in loyalist areas to free up soldiers to take on the IRA in republican strongholds. “It may even be necessary to turn a blind eye to UDA arms when confined to their own area,” Tuzo wrote to the Northern Ireland Secretary, Willie Whitelaw. “Vigilantes, whether UDA or not, should be discreetly encouraged in Protestant areas to reduce the load on the security forces.”

Sinn Féin have successfully presented this alliance as the natural evolution of British counter-insurgency operations in end-of-empire conflicts like Kenya where the British brutally colluded with tribes hostile to the Mau Mau movement.

It is obviously true that in the IRA, the British army and the UDA shared a common enemy. Senior army officers in Northern Ireland believed that working with the UDA was an alliance born of necessity. They did not consider 30,000 troops sufficient to be able to fight a war on two fronts, against not just the IRA but also the loyalists. So the army cosied up to the loyalists even though they had begun to kill Catholics randomly, sometimes in the most sadistic ways imaginable. That did not deter Whitelaw either. The day after two Catholic workers at the Rolls-Royce plant in east Belfast were murdered, he met UDA leaders to seek their help.

The army also tolerated UDA barricades. Thus emboldened by the slack cut them, the UDA stopped some Catholics and took them away for “interrogation” before murdering them in cold blood.

In the hope that loyalist frustration could be channelled into a “constructive and disciplined direction”, the British had established a part-time security force, the Ulster Defence Regiment. UDA members were allowed to join the UDR.  “I am sure that this moderate line towards UDA supporters is the right one in view of the role of the UDA as a safety valve,” Army HQ in Northern Ireland told the Ministry of Defence in London.

Before long, weapons from UDR bases were finding their way into the hands of the UDA and other loyalist paramilitaries like the Ulster Volunteer Force. By early 1973, some 222 guns had gone “missing” from the homes of UDR soldiers, had been “stolen” from UDR armouries or were “lost in transit”. In some areas 15 per cent of UDR soldiers were members of loyalist paramilitaries. By then, there had been some 140 sectarian murders, mostly carried out by the UDA.

Early evidence of this security force-loyalist collusion was gathered by Sean Donlan, a senior Irish diplomat on his trips to the north. Little escaped the notice of Protestant and Catholic neighbours in rural Northern Ireland. Trusted Catholic confidents told Donlan of Protestants who had joined the UDR and were lending their weapons to  paramilitaries. Donlan reported this to MI6 officers based at a British government house outside Holywood in County Down. Even though Northern Ireland was part of the United Kingdom, in those days MI6, the foreign intelligence-gathering service, not MI5, was the lead agency there. Donlan says MI6’s response was, “So be it”. He warned them, “If you don’t deal with this, this little seed will grow.”

And so it did. Between 1973 and 1978, some two dozen serving and former police officers and soldiers actively colluded with the UVF in an area of Armagh and Tyrone that became known as “the murder triangle”. Some operated from a farm at Glenanne, County Armagh, then owned by a part-time policeman. “As well as being a weapons storage base there was other anecdotal intelligence and evidence that showed that his farm was used as a planning centre for several of these attacks,” says Dave Cox, a former Metropolitan Police Commander who headed the recently disbanded  Historical Enquiries Team in Belfast which reviewed the files on some 50 murders of Catholics carried out by members of the so called “Glenanne gang”.