Fifty years ago this week - August 15, 1969 - Roy Hattersley, the junior Defence Minister, had just arrived for lunch in the Gay Hussar restaurant in Soho when Victor Sassie, the amiable proprietor, told him to urgently ring his office.
Crouching in the cramped telephone booth beside the stairs, the Downing Street switchboard connected him to his ministry from where he was told that the Government had agreed to deploy peace-keeping troops to Northern Ireland.
Moments later he was connected to Stormont where Robert Porter, the Home Affairs Minister in the Northern Ireland Parliament, was told that his earlier request for military aid to the civil power had been granted and troops would be on the move within the hour.
The go-ahead was given by Prime Minister Harold Wilson and Home Secretary James Callaghan.
Earlier that morning Callaghan had boarded an RAF plane bound for Cornwall to meet Wilson, who was on holiday in the Scilly Isles, but their plan to discuss the deteriorating security situation in Northern Ireland was foreshadowed by an urgent radio message from Downing Street, routed via Gibraltar to Callaghan's RAF aircraft.
While airborne they created a radio link to Wilson for the pair of ministers to take the critical decision.
The Army deployment had been anticipated for months as the security situation in Belfast deteriorated.
Every morning Wilson cast a rueful eye over the daily intelligence assessments which had crossed his desk since the autumn of 1968 when the first clashes took place.
Over the following months the RUC struggled to cope with the surging waves of protest and sectarian violence throughout Northern Ireland.
But as the civil disorder escalated in the early months of 1969 Harold Wilson was seriously concerned about the lack of intelligence coming from the RUC about the growing violence.
He was horrified to learn that the Special Branch desk in Belfast had only one photocopier at their disposal. Several military officers and more from Security Service MI5 were sent in to reorganise and motivate them.
Wilson had little sympathy for long-standing Ulster Unionist rule in Northern Ireland. As a Civil Service economist during the Second World War, he saw for himself how the Unionist Party gerrymandered electoral boundaries to cling to power and how Catholics suffered from discrimination in jobs and housing.
Like many of his MPs he shared the Labour Party's long-term vision of radical reform to create a more equal society and, ultimately, a united Ireland.
Wilson was also troubled that the RUC didn't have sufficient officers, suitable equipment or training to contain the situation.
Officers sent out to combat rioting had to purchase motorcycle helmets and commandeer bin lids to protect themselves.
Wilson was determined not to involve troops, because he feared it would be misrepresented as sending military might to bolster the unionist cause.
Indeed, he went much further and commissioned two officers from the top of the Army to secretly prepare an assessment of a complete military withdrawal from Northern Ireland.
At that time the military garrison in Northern Ireland consisted of 5,420 Army, 400 Royal Navy, 1,600 RAF and 4,650 of their dependants. An additional 5,070 local civilians were also employed.
There were 45 military locations, including the joint submarine school in Londonderry, the aircraft servicing unit at Sydenham and the armament facility at Antrim, where Mark 8 torpedoes were serviced (they were the primary weapon of the submarine fleet).
The RAF used Ballykelly for long-range maritime reconnaissance, while maritime strike and air defence flights operated from Aldergrove. Air traffic control operated a joint civil and air defence radar installation at Bishopscourt.
The Army had significant bases at Holywood, Kinnegar, Ballykinler and, together with the Territorial Army, were committed to Nato in the event of war.
The study concluded: "There are no direct reasons why the withdrawal of all service facilities would not be feasible."
They calculated that it would take three months to transfer all personnel and dependants, stores, fuel, ammunition, vehicles and equipment.
But, the study warned, funds would have to be provided to replicate some of the facilities in Britain and the pull-out would have to be cleared with Nato.
It added that such an operation would be both cumbersome and embarrassing and would inevitably aggravate the combustible situation in Northern Ireland.
The Top Secret report warned that, if the violence increased after a complete military withdrawal and civil order had collapsed, Britain would face heavy international criticism. In that eventuality, Britain would have to send the soldiers back in again.
Without bases, that would amount to the invasion of a small, hostile country. On Wilson's orders the sensitive report was shelved and he ruled out a military withdrawal.
In August 1969, after the troops had gone in, Callaghan said it had been all too easy to put the troops in, but it would be "the devil of a job to get them out again".
Even in their wildest dreams, Wilson and Callaghan would never have thought that it would be another 37 years before the end of Operation Banner, as the Northern Ireland campaign was called.
Chris Ryder was Sunday Times bureau chief in Belfast during the Troubles. His books include A Special Kind Of Courage: 321 EOD Squadron - Battling The Bombers, The Ulster Defence Regiment: An Instrument Of Peace, and The RUC 1922-2000: A Force Under Fire