LONDON — It had been billed as a civil rights march to redress long-festering hurts, one among many that freckled Europe in the heady days a half-century ago when the streets from Paris to Prague became arenas of revolt.

But that particular protest in Northern Ireland on Oct. 5, 1968, signaled the beginning of something that endured for three decades, seeding an insurgency that became known with weary understatement as the Troubles.

From then until a settlement known as the Good Friday Agreement was signed in 1998, some 3,600 people died in conflict that had all the appearances of civil war, with roadblocks and bomb blasts, sniper fire and the suspension of civil rights.

The British authorities deployed the army against their own citizens in a province that had been carved out as a Protestant enclave at the partition of Ireland in 1921. Protest drawing on centuries of disaffection turned to armed revolt spearheaded by the underground Irish Republican Army and its political wing, Sinn Fein, which cast themselves as the most radical champions of an aggrieved Roman Catholic minority.