At 4.15 p.m. on Thursday, August 14th, 1969, 80 men of the Prince of Wales Own Regiment set out for Derry's Waterloo Place and replaced the RUC.

"Don't make them welcome, they have not come here to help us," Bernadette Devlin called out, but Eddie McAteer spoke to people who had just been in combat with the police and successfully urged them to welcome the soldiers.

Two days earlier, as a parade of 15,000 Apprentice Boys had wound past the perimeter of the Bogside, an intense battle had developed between the police and Catholics.

RUC attempts to remove a barricade had been halted by a deadly rain of petrol-bombs from the high roof of the Rossville flats. Some time after midnight the first canisters of CS gas had been fired, but the wind had blown the gas towards the police.

By mid-morning on August 13th the fighting in Derry had become widespread and sustained. Rosemount RUC station had been set on fire and, while other buildings blazed, mobs resumed their sectarian warfare.

The Taoiseach, Jack Lynch, had made an emotional broadcast which had done nothing to calm the situation, hinting at intervention by saying "the Irish Government can no longer stand by and see innocent people injured and perhaps worse".

Completely over-stretched and utterly exhausted, the police had lost all capacity to control the unrest.

Harold Wilson's government could no longer avoid making the fateful decision: the troops would be sent in. The British army was on active service in the streets of a city in the United Kingdom.

Soldiers had not arrived in time or in sufficient numbers to prevent terrible violence. On Thursday, August 14th, tension steadily increased in Belfast. Here, the very size of the city ensured there were numerous potential flashpoints, often widely separated.

Protestants were angry that whole areas of the region had apparently slipped out of government control and that the victory of the Bogside over the RUC was a deadly blow directed at the integrity of Northern Ireland.

Catholics, in turn, were exultant that the regime so many of them loathed was now thrown on the defensive. Fear was at its most intense along the Falls-Shankill divide.

As night closed in, Protestants supported by some fully-armed B Specials surged down the narrow streets interconnecting the Shankill and the Falls, tossing petrol-bombs into houses as they went, and as they emerged into Divis Street the mobs clashed repeatedly. Fierce fighting also erupted at Ardoyne. Gunfire became general.

From armoured cars police fired bursts of heavy-calibre bullets from their Browning machineguns, fitted as a desperate measure earlier in the day. Many shots hit Divis Flats and a nine-year-old boy was killed as he took refuge in a back room.

The decision to bring troops into Belfast was only taken at midday on Friday, August 15th, and it was not until the late afternoon that the 2nd Battalion the Queen's Regiment moved into the city, followed by men of the Royal Regiment of Wales.

By that time six people had been killed or mortally wounded in the city, at least 12 factories had been destroyed, and over 100 houses had been wrecked and another 300 damaged by petrol-bombs.

Huge barricades had sprung up earlier in the day: telegraph poles, trees, thousands of paving stones, vans, trucks, cars and 60 corporation buses were used to construct them.

Watching a great pall of smoke rising from a burning linen mill, one officer thought it all looked like a second World War newsreel.

Residents on the Falls Road, preferring soldiers to armed police, plied them with cups of tea.

Lieut Gen Sir Ian Freeland, the GOC, nevertheless had the prescience to give this warning before television cameras soon after his men had moved into Belfast: "The honeymoon period between troops and local people is likely to be short-lived. Indeed, it is probably at its height right now."

The Westminster government had no idea it was committing itself to decades of direct military involvement. Jim Callaghan, the Home Secretary, had told the Commons the GOC had been instructed "to take all necessary steps, acting impartially between citizen and citizen, to restore law and order. Troops will be withdrawn as soon as this is accomplished. This is a limited operation."

The start of the Troubles was not an event like the General Strike, which would be dramatic enough while it lasted, but over and done with when it had finished. August 1969 was the gushing to the surface of fears, clashing aspirations and hatreds which had been nurtured over centuries.

The resentment of the Catholic minority, marooned in a devolved region, had festered as successive unionist governments had permitted and often fostered discriminatory employment and electoral practices, and had allowed gerrymandered local authorities to allocate council houses unfairly. Vastly improved educational opportunities, largely financed by Westminster, had created a new highly articulate Catholic middle class, and television had taught viewers the victories that could be won by direct action.

At the same time, growing numbers of Protestants were apprehensive that bridge-building gestures and reforms forced on the Stormont administration by Westminster were weakening the bulwarks of unionism.

Lynch's broadcast had only served to heighten loyalist fear that Wilson's government, in response to Catholic direct action and Dublin interference, would send Northern Ireland sliding towards the horrors of a united Ireland.

No one in the Labour government seems to have had a detailed knowledge of the Northern Ireland situation; Richard Crossman confided to his diary that "we know nothing at all about it".

Within days of committing the British army to the streets, the cabinet was considering how military disengagement might take place. No major political changes were envisaged.

Freeland's prediction was certain to come true because Northern nationalist hostility to British military presence was too ingrained. In any case, no matter how even-handed troops attempted to be, they were not trained police officers and many had got their experience of soldiering during the ignominious intervention in Aden.

The heavy-handed Lower Falls curfew of July 3rd-5th, 1970, hastened the alienation of people in the Catholic ghettos and gave a crucial boost to the confidence and strength of the Provisional IRA.

The troops had been sent to Northern Ireland in aid of the civil power, which meant it was not long before they were seen as agents of the discredited Stormont regime.

Then, in August 1971, by implementing internment they seemed to prove that they were at the beck and call of Brian Faulkner's unionist administration.

The great majority of Catholics had no difficulty in viewing the soldiers as brutal oppressors after Bloody Sunday, January 30th, 1972. Despite the succession of atrocities perpetrated by the Provisionals and the imposition of direct rule thereafter, the standing of the British army in the Catholic enclaves never really recovered.

Conservative and Labour ministers were to admit openly that it was a mistake not to insist on the suspension of Stormont as soon as the troops were sent in. Immediate direct rule would certainly have made the army's job easier, though not in Protestant enclaves.

It is naive, however, to think that Freeland's "honeymoon period" could have lasted for year after year. Ulster's intercommunal quarrel was too deep and too acrid for that.

And what if the troops had not been sent in? Events in the Balkans in the 1990s have shown with awful potency the consequences of untrammelled ethnic conflict. Can it be said with complete confidence that, unrestrained, we Irish in the North would have behaved in a much more civilised fashion?

There are uncomfortable parallels between the background to problems in the former Yugoslavia and to that in Northern Ireland and, after all, the inhabitants of Bosnia - like those of Northern Ireland - are divided not by race and language but solely by different cultural traditions and religious affiliation.

Jonathan Bardon is author of A History of Ulster, The Blackstaff Press, 1992