I am totally committed to the Belfast Agreement but my heart sank when I read Martin McGuinness's polemic against me (The Irish Times, October 29th, 1998) - not because of the sterile rehearsal of old grievances, not because of the highly tendentious view of the peace process given there, but because of a single sentence. Martin McGuinness says: "The agreement is not a peace settlement."

The 85 per cent of the people of this island, North and South, who turned out to vote for the agreement in May have been the victims of a big Sinn Fein joke. We all thought we were voting for a peace settlement. We all thought Sinn Fein had signed up to a peace settlement. Why on earth did we not see Martin McGuinness's hand-written message "this is not a peace settlement"? Because it was not there and is not there.

I simply ask the people of the Republic to reflect on Mr McGuinness's phrase. Perhaps they are entitled to ask themselves, if this was not meant to be a peace settlement, why then the early release of terrorist prisoners? Why then the reduction in security measures in Northern Ireland? Why then the provisions for decommissioning? Perhaps the people of the Republic will understand the difficulties many unionists of good will experience when they read Mr McGuinness's phrase: "The agreement is not a peace settlement."

What does this phrase, "The agreement is not a peace settlement", suggest? It suggests that Sinn Fein's "peace strategy" continues to be a "threat of a return to war" strategy. Mr McGuinness's sentence is the clue to Sinn Fein's current position on decommissioning: "No. Nothing. Never." Such rhetoric creates a suspicion that Sinn Fein is attempting to collapse the agreement by a refusal to meet its decommissioning obligation. A build-up of similar rhetorical tension preceded the Canary Wharf bomb in February 1996.

How is this compatible with the agreement's commitment to end the politics of threat in Northern Ireland? And I ask the people of the Republic: "Is this what they voted for in May, an agreement which is not a peace settlement?" Can they accept Sinn Fein's use of words? No, of course they cannot.

The British government does not accept this use of words. The Irish Government does not accept this use of words. My party does not accept this use of words. Only Sinn Fein accepts this use of words and, in doing so, abuses the meaning of language itself.

Which just goes to show that Sinn Fein has yet to see the writing on the wall. It does not say: "The agreement is not a peace settlement." It says: "The time for hesitation is through. Make the final transition to democratic politics." What does that mean? It means paramilitary groupings proving their willingness to make the transition to democracy on the basis of arms being given up. There is a certain surreal irony, is there not, in the IRA's constant litany of "not an inch" and "what we have we hold"?

When this transition is made, then and only then, is Sinn Fein eligible for ministerial office on the basis of its electoral mandate. As First Minister of Northern Ireland, with responsibility for the democratic integrity of institutions in Northern Ireland, it is my duty to ensure that the line is firmly held on this principle. I have no desire to humiliate republicans but it can only be at this point that the words "an inclusive settlement" can have real meaning.

Throughout the peace process, Sinn Fein persistently assumed that the eventual outcome would embody the "republican agenda". In fact both governments and the democratic parties stuck by the principles of consent and decent compromise. They did not buy into the republican wish list. I have said that rather worryingly Sinn Fein does not appear to have internalised this fact.

They talk of the "British army of occupation" and they still make no distinction between legal and illegal arms. But this is absurd. The agreement insists that Northern Ireland is for nationalist Ireland a legitimate part of the United Kingdom. Above all, Sinn Fein is still demanding that Britain become a persuader for Irish unity. But the agreement rules this out. Why do so many republicans apparently believe that they will be rewarded if their intransigence subverts the deal? Both the Prime Minister and the Taoiseach have already said - nothing better is on offer. This is the only agreement in town and I want to reassure the people of the Republic that I am committed to the pluralist, inclusive Northern Ireland it seeks to deliver. The problem we all have is including in government those who profess brazenly that: "The agreement is not a peace settlement."

For all its blustering arrogance and self-righteousness, for all its excessive protestations and pained outrage, for all its torturing of the language, there appears to be one single revelation in Mr McGuinness's position. It is that Sinn Fein cannot accept the possibility of a new rapprochement between Northern Ireland and the Republic. That is why it seems bent on a wrecking strategy.

Speaking in Edinburgh last week, Mr Ahern confirmed his belief that the agreement, as endorsed by the people of Ireland, North and South, in concurrent referendums is a valid expression of self-determination. I agree with him. Mr McGuinness clearly does not. Mr Ahern remains an Irish nationalist. I remain a convinced Ulster Unionist. This does not prevent us from affirming our commitment to work together on issues of common concern.

I am determined to see an end to the cold war which has plagued relationships between the two parts of our island just as much as I am determined to keep faith with my commitment to build a Northern Ireland at peace with itself. I am certain that Mr Ahern shares the same objective. That is why we want to see the agreement deliver on its potential for stability and security. We want to set in place the real opportunities for co-operation, North and South. Neither of us wants the surreal world of Mr McGuinness where things can be true and untrue at one and the same time.

Within the parameters which the agreement has laid down there is a tremendous potential to make the best of our distinctive and respective contributions to the welfare of everyone on this island. More than that, on the firm foundations of our mutual recognition and respect, both parts of the island have their roles to play in fostering a lobby for the common interests of these islands within the EU. The British-Irish Council is not for me, and I am certain not for the Irish Government, a mere unionist bauble.

That is why I welcome the Taoiseach's comments that we now have the wonderful opportunity to foster "a new spirit of co-operation and friendship between the component parts of these islands". In fact, I believe we are on the verge of an historic breakthrough. I can assure him that I and my party will do everything in our power to foster the new spirit of co-operation and friendship between the two parts of this island. North and South can engage intelligently with one another. I look forward to the continued easing of old and self-diminishing animosities.

I did not lead my colleagues back to the negotiating table in September 1997, did not stay in to the very end when other unionists urged a boycott, did not sign up to the agreement, did not take my party through the pain barrier of prisoner releases, did not allow a situation to arise whereby meetings between myself and the Taoiseach become almost routine, and, above all, did not facilitate four private meetings with the leader of the republican movement, Gerry Adams, in order to pursue some partisan, exclusionist agenda.

But, like Mr Ahern, I and my party are frustrated by the obstacles put in the way of progress by those who continue to say: "The agreement is not a peace settlement", who by their very words threaten the promise of the agreement by recourse to war.

David Trimble MP is First Minister of Northern Ireland and leader of the Ulster Unionist Party.