THE Aug. 31 truce of the Provisional Irish Republican Army is a milestone in a bitter struggle going back to England's conquest of Ireland in the early Middle Ages. Divid-ing the island in 1921 into an independent state and a province of the United Kingdom proved an ephemeral solution. For 25 years, Ulster has been a war zone for extremists. History suggests, as does the radical Protestant backlash to the cease-fire, that lasting peace may be elusive.
The present phase of Ulster's ``Troubles'' arose out of civil-rights demonstrations in the late 1960s to better the lot of Northern Ireland's Catholics, who make up nearly 40 percent of the 1.6 million population.
In 1969, clashes between civil-rights demonstrators and Ulster police prompted Britain to deploy soldiers to the region. But violence worsened, increasing sharply after Jan. 30, 1972, or ``Bloody Sunday,'' when British troops confronted a prohibited Catholic civil rights march and killed 13 civilians.
Attempting to contain the violence, Britain later that year reimposed direct rule over Northern Ireland.
The IRA, formed in 1969 to counter the presence of British troops in Ulster, continued its terrorist campaign for unification with the Irish Republic. Protestant paramilitary groups responded in kind. In recent years, they have killed more people than the IRA.
The main issue in Ulster is allegiance. The Protestant majority overwhelmingly favors ties to Britain. Most Catholics advocate peaceful Irish unity, though some wish to retain British ties on economic grounds.
The seeds of the current cease-fire were sown by a tacit admission from the IRA and Britain that neither side could win militarily in a conflict that has claimed more than 3,000 lives.
Until recently, the moderate Social Democratic and Labour Party, which advocates peaceful Irish unity, seemed to have little in common with the outlawed IRA. But in 1993 its leader, John Hume, courageously held prolonged talks with Gerry Adams, president of the IRA's political wing, Sinn Fein.
On Dec. 15, 1993, Britain and Ireland announced the Downing Street Declaration, superseding the dramatic 1985 Anglo-Irish Agreement. In the earlier accord, Britain had agreed to Dublin's involvement in running Ulster, in return for Dublin's recognition of Northern Ireland's right to exist.
The Downing Street Declaration went still further. It underscored that Britian and Ireland had a joint purpose in seeking peace in Northern Ireland, and that Britain had no political or strategic interest in remaining in Ulster. The accord maintains, however, that Ulster will remain a part of the United Kingdom as long as a majority in the province votes to keep it so.
While Protestant and Catholic attacks continued, the IRA waited months to give a verdict on the Declaration. But recent ``clarification'' may have assured Sinn Fein and the IRA that Irish unity is ultimately inevitable, and led the IRA on Aug. 31 to announce an end to its war against British rule.
The IRA still must prove, over a period of months, that it has permanently renounced violence before Sinn Fein may join peace talks. But radical unionists, fearing that Britain may be deserting Ulster, immediately tested the IRA's truce with attacks of their own. The future rests in the hands of moderates, who must keep both sides talking rather than fighting.