The Belfast Agreement makes explicit commitment to the achievement of reconciliation, tolerance and mutual trust. There is an inevitable cynical reaction against well-worn phrases. After all, the reconciliation promised by the Anglo-Irish Agreement of 1985 was nowhere to be seen in the last 13 years.

But this time is different: the parties of the North have agreed, and the people, North and South, have supported them. Reconciliation, tolerance and mutual trust are an essential part of the project.

Why recall this now? The reason is simple. In the midst of all the genuine condemnation in Dublin of the Omagh crime, one could not help noticing a disturbing theme creeping through. This was the argument that it was not necessary to carry out this bombing because a purely political means (the Adams strategy) had been found of isolating and marginalising the unionists.

It was an argument which cut against the logic of the agreement, and was as unattractive as those unionist politicians who seemed to think that somehow Mo Mowlam, Tony Blair or the Northern Ireland Office was responsible for the tragedy at Omagh.

There is at the moment a lot of concentration on the new security measures announced by the Government. This is understandable in the context. But the most important message Mr Ahern can send out at this moment is a purely political one. He expressed it clearly in an interview early in the year, when he criticised those republicans who hoped to win a more radical settlement than the one embodied in the agreement.

"So people are saying, `Hang out for a few years'. We will hang out for what for a few years? What process are they talking about? If we have spent 10 or 15 years to get this one right, I do not see their argument. They miss the point that we have followed the same track for a decade. They think we have been on a different track each year of the debate. And they are wrong."

This is above all the message which has to be reiterated today. The fact is that the activities of dissident and fundamentalist republicans had a political logic, to destabilise the agreement by provoking a unionist withdrawal of support. In this they shared a common ground, although not a common tactic, with some of Mr Adams's supporters. They may yet succeed.

As yet the unionist community, like the nationalist community in Northern Ireland, is in a state of shock. Glengall Street is not yet being flooded with calls demanding that David Trimble resign or turn his back on the agreement, but key anti-agreement politicians clearly see their opportunity to raise the stakes on decommissioning, and few can deny the enhanced appeal of that issue for unionists.

The danger is that the emotions this arouses could place intolerable burdens on the implementation of the agreement, especially the provisions for the establishment of an inclusive government including Sinn Fein over the next few weeks.

The Irish Government has, however, the resources to deal with this. It has to spell out, as Seamus Mallon has already done, that this agreement represents a historic compromise for a generation. Mr Ahern has to explain to republicans that if the agreement fails because of the burden placed on Mr Trimble by republican violence, the Government cannot guarantee to deliver a better deal at a later date.

In short, it should be made clear that everyone, not just unionists but Sinn Fein in particular, has an interest in making the type of compromises which will allow the agreement to work.

The position for Mr Adams is admirably unambiguous: fundamentalist militarists will never be weaker. It is obvious that throughout the summer he has lost people to the dissident groups, but any waverers who decided for the time being to stay with Mr Adams will now be greatly relieved.

It will never be easier for the mainstream republican leadership to make the kind of gestures which show that not only do they condemn the Omagh atrocity but they will never again stand over volunteers from whatever section of the republican movement who go out on missions which inevitably over time result in such calamities. Mr Adams has the chance to make that break with the past which John Hume has long urged upon him.

In short, these are tragic times which contain elements both of great opportunity and great danger. The British and Irish governments have to signal that the days of merely tactical or opportunistic adherence to the agreement are over.

They should remind us, as Liz O'Donnell did so forcefully on Sunday, that decommissioning is an integral part of this agreement. They have to point out that nobody will be rewarded for making it difficult for the other side to uphold its part of the bargain. If they act in this way, there is just a chance that Omagh really will signal the Troubles have ended.

It is perfectly clear from the way Mr Trimble is carrying himself that he intends to work the agreement. It is up to the two governments to give him the chance to finish the job.

Paul Bew is Professor of Irish Politics at Queen's University Belfast