Leading article: Espionage as conspiracy and cock-up

Every time Northern Ireland looks as though it might settle down, some new destabilising convulsion erupts to replenish the already brimming reservoirs of suspicion and distrust. The past 12 months have brought significant progress, such as the decommissioning of the guns of the IRA. But they also brought IRA actions such as the killing of Robert McCartney and the robbery of the Northern Bank in Belfast.

The latest detonation is the unmasking as a Special Branch spy of Denis Donaldson, one of Gerry Adams's most trusted apparatchiks. As one of the most mysterious of the many mysteries of the troubles, it cries out for explanation and clarification.

Only fragments of the episode have reached the public domain, and the cynical, though probably accurate, view is that the whole truth will never emerge. The authorities, though, should surely make an effort to provide an account of what has been going on - even a partial account would be better than none.

The world of informers and agents is fascinating but distasteful. Not so long ago, it used to be lethal as well. In the old days, Donaldson's IRA associates would have bundled him briskly down to south Armagh, conducted an interrogation, then shot him in the back of the head. In the new unarmed republican dispensation, he has escaped with his life. The downside for him is that he will have to leave Ireland and will go down in republican history as a traitor.

He will leave behind far more questions than answers. His recruitment as an agent two decades ago was a major coup for the intelligence agencies, and over the years his information must have been hugely valuable. But his spying career has ended in a fiasco which has all the hallmarks of conspiracy, potently laced with cock-up.

His arrest in 2002 may well have broken up an IRA surveillance operation on local parties and on the British government, but it did so at great cost. Political trust collapsed completely and the Belfast Assembly was suspended; it has been in mothballs ever since. Millions of pounds was spent to re-house people whose personal details were uncovered. Donaldson's value as an informant came to a precipitate end.

Everyone knows that the authorities were spying on republicans, and that republicans were spying on them: there was a war on. But now it may be that some intelligence people, unintentionally or deliberately, stalled a peace process and brought down a government.

Tony Blair has been toiling patiently to put that government back together again, and his task has just been made more difficult. He must now establish what happened, and then provide the rest of us with an explanation.