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It follows three days and two nights of violence in the mainly-Catholic Bogside area of Londonderry. Trouble has also erupted in Belfast and other towns across Northern Ireland.
It also comes after a speech by the Prime Minister of the Irish Republic, Jack Lynch, regarded by many as "outrageous interference" in which he called for a United Nations peacekeeping force to be sent to the province.
He also called for Anglo-Irish talks on the future of Northern Ireland.
The Prime Minister of Northern Ireland, Major James Chichester-Clark, responded by saying neighbourly relations with the Republic were at an end and that British troops were being called in.
The British Home Secretary James Callaghan was in a plane on his way to talks with Prime Minister Harold Wilson in Cornwall when he received a radio-telephone call asking for troops to be deployed.
Shortly after 1700 hours local time, 300 troops from the 1st Battalion, Prince of Wales's Own Regiment of Yorkshire, occupied the centre of Londonderry, replacing the exhausted police officers who had been patrolling the cordons around the Bogside.
They have been on standby for the past couple of days.
The arrival of the British troops was greeted with cheering and singing from behind the barricades in the Roman Catholic area of Londonderry.
They were chanting: "We've won, we've won. We've brought down the government."
The trouble began three days ago during the annual Apprentice Boys march, which marks the 13 boy supporters of William of Orange who defended Londonderry against the forces of the Catholic King James II in 1688.
The Royal Ulster Constabulary were forced to use tear gas - for the first time in their history - to try to bring the rioting under control.
But tensions mounted with the mobilisation of the B Specials. The special constables, who are armed and mostly part-time, were supposed to help the RUC restore order - but they are regarded with deep suspicion by the Roman Catholics.
On the streets of Belfast, the appearance of the B Specials led to an escalation in the violence while the special constables reportedly stood by and watched,
The army's warm welcome was short-lived, as was the British Government's intention to pull out the troops within days.
It soon became clear the violence was not going to end.
As more British troops were deployed in Northern Ireland, fresh questions were raised about the role of Westminster.
Although the army in Northern Ireland came under the control of the Secretary of State for Defence in London many Catholics saw it as a tool of the Unionist Government in Stormont.
The violence increased, internment was introduced in August 1971 and on 24 March 1972 the British Prime Minister Edward Heath suspended Stormont and direct rule was reimposed.
It was not until the Downing Street Declaration of 1993 that there appeared to be any real prospect of peace.
After the Good Friday Agreement in 1998 terms were reached to reduce the number of troops in Northern Ireland.
In November 2004 there were 11,000 British soldiers in Northern Ireland - down from a peak figure of about 30,000 in the mid 1970s.
It is planned to reduce the force by a further 6,000 by the summer of 2006.
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