LONDON — A few days before the 20th anniversary of sending British troops to Northern Ireland (Aug. 14), workmen began digging a hole near the entrance of 10 Downing Street. They were installing a reinforced metal security barrier designed to flip up and block the path of terrorists trying to storm the official London residence of Prime Minister Margaret Thatcher. The barrier is a symbol of the continuing threat posed by the illegal Irish Republican Army. Mrs. Thatcher has been an IRA target in the past, and security advisers know another attack could come any time.
The new barrier also underlines a change in British attitudes: Downing Street always used to be ``open territory'' for people wanting to gaze at the plain black door behind which British government is conducted. It now comes close to being a no-go area for all except officials and authorized visitors to No. 10.
Thatcher was out of the country on Monday, the 20th anniversary of the dispatch of British troops to the troubled province of Northern Ireland. At the time of this writing, nationalist protests marking the anniversary in Ulster have been restrained.
If there were hopes in 1969 that the soldiers could bring peace to Northern Ireland, they have been dashed. The best that can be said is that the presence of 10,000 troops in Ulster has helped contain the problem.
Security sources say that a mere 300 to 400 IRA members are active terrorists. The huge British military presence, supported by 6,000 members of the local Ulster Defense Regiment, is required to preserve law and order in the province.
According to one British official, there is no prospect of reducing the numbers of security forces any time soon. ``The British presence is the difference between relative order and certain chaos,'' the official said. That is Thatcher's view as well.
It is also the opinion of the nine British politicians who, since 1969, have held the post of secretary of state for Northern Ireland. All, having done the job, must be accompanied by a Scotland Yard detective for years afterward. They, too, are top terrorist targets.
In Northern Ireland the religious and social equation has changed hardly at all. A Roman Catholic minority shares six counties with a Protestant majority which, with support from London, refuses to consider uniting Northern Ireland with the Republic of Ireland.
There are other aspects of the equation, and they add to the cost of maintaining the status quo in Northern Ireland. Because unemployment is high (double the mainland rate) London pumps in $2.4 billion a year to keep the economy afloat. Roughly half the jobs in the province are financed through government spending.
The worst part of the equation, of course, is the continuing loss of human life. Since 1969, 2,762 people have died in the troubles - 2,167 of them civilians.
Thatcher has sworn that she will give not an inch to the IRA. British officials will say publicly that the troops are in Northern Ireland to defeat the IRA. Pressed hard, however, they often concede that the best that can be hoped for is the containment of terror.
Open rioting between the two communities in Belfast has diminished, as has the number of bombings and shootings. But the IRA has a higher success rate with the targets it does attack.
The security authorities' worst nightmare is that the IRA may begin to deploy modern ground-to-air missiles against the helicopters used for reconnaissance and for troop transport. So far the nightmare has been held at bay, despite reports that the IRA has some SAM-7 missiles and has been trying to obtain Stingers.
When Thatcher returns from her holiday, the battle to check terrorism will continue. Much faith is put in the 1985 Anglo-Irish accord between London and Dublin. It has helped to tighten up cross-border terrorism, and provides a venue for top-level political consultation.
But the prospect of defeating violence in the short term does not exist. One British official summed up the mood: ``There is nothing to celebrate after 20 years, except that the terrorists have not prevailed. There is no way of knowing how much longer our troops will be needed there. They will stay for as long as necessary.''