Two years after the Summer of Love, Northern Ireland made the case for hate.

By August 1969, sectarian turmoil over the campaign for civil rights had spread to the extent that the RUC could not control Londonderry or parts of Belfast.

To deal with the unrest, the Westminster Government agreed to deploy Army troops garrisoned in Northern Ireland at that time - less than 2,000 - supplemented by a battalion of light infantry from Plymouth.

That was the start. It was an emergency measure that quickly seemed anything but temporary; lasting 38 years, ending tonight, during which more than a quarter of million members of the regular Army were brought to the streets of Belfast and sheughs of Armagh and Tyrone. More than 700 servicemen were killed by paramilitaries.

The Army famously arrived to cups of tea on the Falls Road; it has already long since left that republican heartland and gradually reduced its role so that tonight's transition has more the air of a shift change than the radical transformation it represents.

Northern Ireland became one of the defining theatres for the British Army: its upper tier of commanders gained much of their operational experience here - former Chief of the General Staff Sir Mike Jackson was a Para officer on Bloody Sunday - and the Army acquired an international reputation for policing civilian strife. Some here would argue that reputation was undeserved, because the other side of the Army experience ranged from " harassment" - the apparently arbitrary use of power - to the use of ruthless, possibly extra-judicial, covert measures.

None of that was envisioned when the troops began arriving in 1969. But what began as a support operation for the RUC was turned into something altogether more significant as political events took their course.

While the cups of tea were offered in front of the TV cameras in the initial relief from the sectarian battles of '69, the Army's relationship with the nationalist population was perhaps not as relaxed as those images suggested.

Unrest continued through the next year, but a milestone in what was to follow happened at the start of July, when an Army operation - through accident or by design - expanded into a full scale clampdown and search known as the Falls Curfew. Guns, explosives and ammunition were found, but the event significantly alienated much of the nationalist population in the city.

Violence escalated over the next few months, and in February 1971 the Army lost its first soldier in the streets. Gunner Robert Curtis was shot by an IRA sniper in the New Lodge, in North Belfast. He was 20.

Over the next year, as the more militant Provisional IRA began to wrest control of republicanism from the Official IRA, the Army was increasingly at the centre of violence. Its responses could range from the horrific to the heroic.

Typical was the killing of William Halligan, a 21-year-old Catholic shot dead by the Army 27 days after Gunner Curtis died. Halligan was shot in disputed circumstances in the lower Falls, with soldiers insisting he had been about to throw a bomb. Residents of the area said he was shot for taunting soldiers. Eight years later his family received damages from the Ministry of Defence.

A sergeant leading the unit that shot William Halligan had just over two months to live. Michael Willets was in Springfield Road RUC station when the IRA threw a suitcase bomb into the reception area, where at least two children were waiting with their parents. Sgt Willets pushed the children into a corner and shielded them from the blast. He was posthumously awarded the George Cross.

The next year saw Willets' regiment, the Paras, involved in one of the most notorious events of the Troubles: Bloody Sunday, the shootings that left 14 people dead in Derry. The Army officially continues to insist that the shootings were prompted by a shot being fired at soldiers, but the backlash was to fuel the existing instability. 1972 became the single worst year of the Troubles, with 496 people dying, more than a fifth of them soldiers.

The Army views this period as a "classic insurgency", marked by gun battles in the streets. But by the mid-Seventies, changes in the approach of both the IRA and the military changed the character of the conflict into a settled, long-term campaign.

"PIRA developed into what will be probably be seen as one of the most effective terrorist organisations in history," said the Army's official account of Operation Banner. The Provos' effectiveness in this new role - carefully planned, isolated attacks - was lethally demonstrated to the Army on August 27, 1979. Eighteen soldiers were killed in a two-stage bomb attack at Narrow Water Castle, outside Warrenpoint. It was the worst single incident for the Army throughout the Troubles.

The Army's role changed, too - the SAS was officially deployed in 1976 and the military focus became, again in the Army's words, "the destruction of PIRA, rather than resolving the conflict".

The shoot-to-kill era in the 1980s provoked extreme controversy, peaking in 1987 with the Loughgall ambush that killed eight IRA men and a civilian. The Army believes this tactic of using intelligence to catch or ambush IRA men as they retrieved weapons or began attacks was highly effective.

"PIRA seem to have been brought to believe that there was no answer to Army covert operations, and they would not win through violence," the Banner history says. "That was probably a key factor."

The Provos' development of their "peace strategy" led to the 1994 ceasefire, and a gradually changing role for the military. The decline in violence was not cost-free, however. The lapse of the IRA ceasefire in 1996 and 1997 saw a bomb attack at the Army's HQ, Thiepval Barracks, that killed Warrant Officer James Bradwell and reminded everyone of how deadly the IRA could be. In February 1997, 23-year-old Lance Bombardier Stephen Restorick became the last soldier to die, cut down the same way as the first, Gunner Curtis - shot by a sniper in the last spasms of the Troubles.