Differences that appear small, minimal even, to an outsider can be momentous from within. Just 126 paces separated the route that the Shankill Protestant Boys and the Sons of Ulster wanted to march along this weekend, and the route they were allowed to take by the Northern Ireland Parades Commission. The ruling was the excuse for three days, so far, of the worst rioting that Belfast has seen for years. Police officers have had to brave petrol bombs, rocks and gunfire. Violence has spread to loyalist areas of other towns. Many of those hurling the missiles have been children. It is a reversion to grim days.

There is never an excuse for such violence, least of all when the pretext is a rerouted parade. And that is just what it was, a pretext. It was “loyalists” against the Police Service of Northern Ireland and British troops. This is a disturbing and potentially destabilising moment. There is plenty of evidence to suggest that the violence was preplanned. The hand of Ulster’s banned paramilitary groups, the Ulster Defence Association and the Ulster Volunteer Force, is apparent. Equally worrying is the way that the Orange Order has unwittingly or wilfully allowed itself to be dragged into the fray. This is an alliance that must not be allowed to stand. The aim of the paramilitaries is to extend their support from “loyalism’s” wider fringes in East Belfast into the Unionist mainstream. It is imperative that Unionist leaders stand up to this threat.

Unionism is showing signs of dangerous instability. Political concessions to Sinn Fein and the IRA have convinced many Unionists, wrongly, that the political process holds no promise for them. It is understandable that some loyalists feel they have been bypassed by any dividends from the peace process. Many lives have not improved economically or socially, and such deprivation inevitably fuels anger.

These are testing days for Ulster. The wrong move could do untold damage to the peace process. This makes it vital for the authorities to tread with firmness but also with care. Peter Hain, the Northern Ireland Secretary, is right to take a deep breath before pronouncing whether the UVF has breached its ceasefire. IRA decommissioning is imminent. The last thing that the peace process needs is for “loyalist” gunmen to run amok. The current violence must become the death rattle of Unionist extremism rather than the opening of a new chapter of horror. At all costs, the brutalising of another generation must be avoided.