Here is a grim scenario for 1999. Even after the elation of the recent successful deal with the SDLP on the final shape of cross-Border bodies, it still haunts the Ulster Unionist Party. Most of the Assembly party is solidly behind Mr Trimble but the party is bleeding gently at the grassroots. There is little basis for a mass revolt but hard-working cadres continue to slip away, repelled by the compromises which are the motor of the Belfast Agreement. Drumcree remains unresolved and there are growing fears about the future of the RUC.

In the European election of June 1999 - probably boosted by a "middle Ulster" perception that Tony Blair's pledges on decommissioning are of little value - Ian Paisley, supported by Robert McCartney, humiliates Jim Nicholson, the Ulster Unionist candidate, electorally and proclaims that the people, at any rate the unionist people, have now turned against the agreement.

To complete the grim picture, John Hume finally - as is indeed highly likely - tops the polls overall for the first time. A rejuvenated nationalism would confront a unionism more bitterly divided than ever before; the No camp would be doing enough to destroy David Trimble's pro-agreement strategy while visibly failing to redress political unionism's relative decline.

Putting the nightmare like this may help to explain the Ulster Unionist position on decommissioning: many sympathetic people outside the ranks of unionism find its attitude frustrating.

They fully sympathise with the rationale: the agreement's commitment to a "new" politics - a politics without threat - which heavily implies decommissioning by parties which are in government.

But it could be said that by placing such emphasis on prior decommissioning, it is letting Sinn Fein off the hook. After all, the republican movement - and both governments regard it as a unified movement - has yet to say that it will decommission within two years, as mentioned in the agreement.

Surely republicans would be vulnerable if the debate shifted to this topic? There is clearly some substance in such an argument but the current UUP position on decommissioning has a deeper logic, the logic of political survival.

The one card the Trimble leadership has against the malcontents and disgruntled is the hope of a relatively clear political future in which violence no longer calls the tune.

The agreement has to be seen as a watershed. Its whole essence is that, whatever the difficulties, there will never be a recourse to violence. It is about burying, once and for all, the threat to use violence to achieve political ends.

IF TRIMBLE is able to defend this principle in the coming months he will remain the dominant figure within unionism. If he loses his battle - or, to put it another way, if the British and Irish governments allow him to lose his battle - then politics in the North will revert to type, with negative consequences for both North and South.

But there is, of course, another more benign scenario for 1999. The conclusion of the Trimble-Mallon deal has put relationships between the SDLP and the UUP back on an even keel.

In some ways, the bungled nature of the first negotiation intensified the pleasure of the eventual agreement. Errors made early in the month were not repeated - this time Trimble's Assembly party was kept fully informed.

This time the interventions made by the two governments were more controlled and precise. Tourism, which for so long threatened the agreement, was finally dealt with in a sensitive way.

This time, above all, Sinn Fein seemed out of its depth - engaging in a hollow PR exercise: aggressively denouncing the unionists for delay and then, having belatedly realised that a deal was shaping up, implausibly giving the credit to late interventions by Sinn Fein and/or the two governments.

The SDLP and the UUP were the centre of gravity for the agreement, and now for the December package; this can only be a good omen for the working of the new model of government in Northern Ireland.

The North is poised between these two outcomes. In David Trimble's conception, the Belfast Agreement has the potential to be the beginning of the end of the bitterness between the Catholics and Protestants in the North: moreover, at this time it has the capacity to create a more relaxed atmosphere throughout the island of Ireland.

But he is bound to fail without support from public opinion in the South. In recent weeks, Trimble has laid out his cards. In a series of speeches in Dublin, Wicklow and Oslo he has acknowledged the success of the Irish economy: the commitment of the Irish State to the principle of consent, and he has acknowledged that Ulster unionists `built a cold house for Catholics'.

He has been careful to place all this in a realistic historical context, but the effort is all the more impressive for that. If the reaction of mainstream Irish public opinion is simply to shrug, "pocket the gains", and carry on attempting to squeeze unionism, then David Trimble will not be First Minister next Christmas.

As Martin Mansergh, Mr Ahern's special adviser, generously acknowledged in his Wicklow address to the Irish Association, the agreement means giving political space to those who wish to construct "a pluralist parliament for a pluralist people" in the North. The day of tired rhetoric about failed political entities has gone forever.

Yet the fact remains that the agreement has provoked remarkably little reflection on the nature of unionism within nationalist Ireland. Again and again, an exaggerated emphasis on the role of pressure from Tony Blair has substituted for more serious thought.

In particular, the question is not asked: if unionism is simply a creed of irrational supremacism, why did a slender majority of unionists support the agreement, equality agenda included, in the referendum?

The British government is signalling that a great deal now depends on the Irish Government's dialogue with republicanism in the North. Bertie Ahern has indicated his distaste for the rhetoric of "not a single bullet": with the completion of the December package, the republican movement is tantalisingly close to a place in the government of the North. Its electoral mandate gives Sinn Fein every right to such a place: only the refusal to do anything about the politics of latent threat is a barrier.

SEVENTY FIVE years ago, republicans blew up the Dublin house of Stephen Gwynn, the former Galway nationalist MP. His "crime" had been to write articles which showed a degree of sympathy for unionism. Gwynn reacted philosophically: he described one of the republicans who had helped his daughter remove some items of sentimental value from the house as "a probably decent little boy who is ordered to do dirty jobs". Gwynn even took the loss of his library in his stride: "Books suffer very little."

The attack did not stop his writing: he continued to insist that scrupulous respect for the rights of minorities in both parts of Ireland was the only way forward. After a long delay, for which unionists in significant measure must share the blame, we have the chance in 1999 to renew such a project.

Paul Bew is professor of Irish politics at Queen's University Belfast.

Tomorrow: John Palmer on Ireland's changing relationship with the EU.