The first thing that Tony Blair and Bertie Ahern must do when they meet in London on Tuesday is to start repairing trust. Not trust between the governments, which work well together, but between those among whom trust is most badly needed, the Northern parties committed to the Belfast Agreement.
Mr Blair and Mr Ahern now know that, with the best will in the world, the governments alone, or by imposition, can't implement the agreement.
They can help and encourage parties; they can try to persuade them to co-operate with each other; they can set the stage on which co-operation occurs. But they can't force them to act against their own, or their supporters', judgments.
The governments may have learned this lesson. At the end of a frustrating week, they've wisely decided to appeal once more to George Mitchell for help. He has the qualities and experience they need for a sharp review of the agreement's implementation.
His major report was a model of clarity. His deep local knowledge, painfully accumulated over two years at close quarters with parties and paramilitaries of all shades, impressed the locals in a city of sceptics. But he could hardly be blamed if, on viewing the week's events, he were to set strict limits on his availability for yet another tour of duty in Belfast.
He will be familiar with the arguments which preceded and followed the negotiation of the Belfast Agreement. But if he needed a reminder of older, deeper bitterness, he will find it in the records of Thursday's session of the Aseembly.
There, in the absence of the Ulster Unionists, the DUP and Sinn Fein engaged in a series of exchanges which could scarcely be called debate.
Ian Paisley, Gerry Adams and their colleagues traded sectarian hostility and contempt in an atmosphere from which, perversely, both sides appeared to draw some satisfaction.
Trust, which is essential to politics and to relations between parties, was - like the UUP - missing from the chamber; mistrust hung in the air like blight.
BY comparison, David Trimble's message to Sinn Fein was both friendly and conciliatory. "We cannot work without you," he said, "but we cannot work without decommissioning."
It's a point which was made repeatedly this week and one that's bound to feature largely in the planned review. Decommissioning is a major source of the present difficulties and could prove an obstacle to eventual agreement.
For most unionists - and not only the members of the UUP - trust is shorthand for decommissioning, and until the issue is resolved mistrust will continue.
David Ervine of the Progressive Unionist Party takes a different view, choosing to regard decommissioning as an issue within unionism rather than one between unionists and republicans.
There is no equivalent issue on the republican side, but both republicans and nationalists say they sense, in the UUP's attitude to the all-party executive, a reluctance to acknowledge equality and share power.
The unionists' response is that they couldn't trust the IRA to begin decommissioning if an all-party executive were set up.
They'd been saying so for months, but up to this week the governments believed they could persuade David Trimble to change his mind.
He made the case that there was no commitment to decommissioning by the IRA; Sinn Fein did not agree to a timetable to complete decommissioning by May 2000 and the SDLP, though impatient with the republicans, refused to say in plain language that it would proceed without Sinn Fein in the event of default.
In an interview after his resignation, decommissioning was the issue which Seamus Mallon specified as on the agenda for a review.
When Mo Mowlam announced Mr Mallon's resignation in the House of Commons, most of the questions from the Conservative spokesman, Andrew McKay, and others were on decommissioning.
She said the review would not cover the agreement as a whole but its implementation and, as she reminded MPs of advances made in the past five years, she warned that the last thing the people of Northern Ireland needed now was an outbreak of recrimination: "We move forward together or not at all."
One of the changes is that the republican movement has moved from the position in which Britain was the enemy, responsible for everything, culpable on all occasions.
Tony Blair, after all, has devoted more time, energy and political resources to the North than any British prime minister in the last 70 years.
NOW, in the eyes of republicans, unionism is the enemy, irredeemable, immutable, incapable of reconstruction. Most of all, say republicans, unionism is unwilling to share power.
But unionism is one of the isms that won't go away, certainly not for a long time, perhaps not at any time.
Much was made of the fact that the UUP had stayed away from the Assembly on Thursday morning.
Two reasons were given for that: the party didn't want to be embroiled in the DUP's attempt to exclude Sinn Fein from the executive. And, of course, it wanted to demonstrate its independence, to show that it couldn't be bounced, as Mr Trimble put it.
Now, the review initiated by the governments will give all of the parties which are committed to the agreement an opportunity to demonstrate their commitment.
Mr Trimble has asserted his leadership of the UUP and of unionism. Those outside Northern Ireland who had been inclined to lump all of unionism together and ignore the challenge posed by Mr Paisley have been made aware of the complexity - and the ferocity - of the competition.
We have been reminded, in the Assembly of all places, what fate might hold if the agreement were to fail and politics in the North were to be reduced to old enmities.
Indeed, in the House of Lords on Wednesday night some of the old hands explained how politics in Northern Ireland had come to its present pass.
Lord Molyneaux assured colleagues that the leadership of the IRA in the southern part of Northern Ireland - south Armagh - had already decided there would be no decommissioning, now or at any time.
Lord Molyneaux himself has been one of Mr Trimble's critics and a sceptic as far as the agreement is concerned.
Lord Fitt said unionists, who brooked no opposition in the 1960s, were now splintered beyond repair. But on the other side, John Hume, who had raised republicans from the political gutter, would not desert them now.
Old memories and lack of trust are the cause of this week's trouble, which finds politicians in Belfast more eager to return to the Boyne than to cross the Rubicon.