"Did David Trimble conclude the agreement on Good Friday? Does he intend to deliver?"

The questions were not posed in the dark days of December as the parties moved gracelessly and grudgingly to agreement on the shape of the putative Northern Ireland Executive and the designation and remit of cross-Border implementation bodies.

Rather they appeared in this newspaper in the heady days of June, as the people of Northern Ireland warmed to the early summer sun and the promise of a new beginning under the terms of the Belfast Agreement.

And they reflected a rising tide of nationalist concern that Mr Trimble - Ulster Unionist leader and First Minister-in-waiting - had effectively reopened the negotiation under the heat of the referendum and Assembly election campaigns.

Enthusiasts for the North's "new" politics had little time then for such dour reflection. Nor was it difficult to see their point. Barely two months before, Mr Trimble had emerged the surprise hero of the hour. We awoke on the morning of April 9th to the agonising question - David Trimble: history-maker or historical footnote? Late the following afternoon he delivered his answer. Losing Jeffrey Donaldson along the way, he challenged doubting colleagues: "I'm going for it. Who's coming with me?"

Totally unprepared, many Ulster Unionists reeled in disbelief. But it was a moment of sweet vindication for David Ervine and others in the PUP. They had stuck with the process, convinced that Trimble understood he would have one chance at a deal. After all the posturing and hard bargaining, it seemed the UUP leader had come to believe the war could actually be over. The final compromise satisfied the Taoiseach's demand that the cross-Border bodies be rooted in London and Dublin legislation - while satisfying Trimble's need to have them in effect mandated by, and answerable to, the Assembly and the Dail. There would, after all, be an executive as demanded by the SDLP. And, more, Mr Trimble rejected a proposal from Lord Alderdice which would have Alliance, the SDLP and the Ulster Unionists carve up the ministerial spoils. Assuming Sinn Fein won the necessary electoral support, Mr Adams and Mr McGuinness or their nominees would be sitting at the cabinet table.

A week later, still stunned, over 70 per cent of the delegates to the Ulster Unionist Council backed the Belfast Agreement. The psychological advantage was with Mr Trimble. But not for long.

The anti-agreement unionist campaigners skilfully identified unionist antipathy to the proposals for an independent commission on the RUC and, in particular, to the two-year programme of prisoner releases. And the effectiveness of the anti-agreement campaign was reinforced by the mismanagement of Mr Trimble's own.

Private polling evidence that the May referendum might be lost brought Tony Blair's direct intervention. The effect was mounting unease in nationalist circles. Mr Blair sought to assuage unionist anxieties with a series of pledges, establishing among them clear intended "linkage" between prisoner releases and Sinn Fein seats in government with the decommissioning of paramilitary weapons.

As Seamus Mallon would later observe, nationalists "bit their lip" after three Blair visits to "bail out" the unionist campaign, going "as close as he could to changing the terms of the agreement without actually doing so".

But the deputy SDLP leader finally broke silence in the final days of the Assembly campaign, warning that a "petulant" David Trimble would have to "reinvent himself again." Specifically, Mr Mallon challenged Mr Trimble's interpretation of the transitional arrangements to come into play before the actual transfer of powers to the Assembly.

In an interview with The Irish Times, Mr Trimble said the "shadow executive" need not come into being until close to Christmas "if then". Mr Mallon insisted it should be established "immediately" after the Assembly election, by which he meant mid-July. Overall Mr Trimble's view prevailed. As Christmas drew ever closer, Mr Trimble was still insistent Sinn Fein's participation in the Executive was and is conditional on a start to IRA decommissioning.

Mr Trimble put down plenty of markers along the electoral way. And many republicans remain convinced that his "engagement" never really moved beyond the tactical and strategic.

It is equally arguable that - having made the leap of imagination on Good Friday - the UUP leader might have conducted matters differently had he won convincingly in the Assembly contest. But it's all academic. He didn't.

Mr Trimble's difficulty, as Gerry Adams once put it, is that he seeks to reintroduce the precondition Mr Donaldson failed to get on Good Friday. Most parties to the Agreement, and most commentators (this one included), agree that - on a literal and legalistic read - Sinn Fein has it right. Decommissioning is not stipulated as a precondition for the party's entry to the Executive. Nor indeed, as Mr McCartney would argue, is there even a defined penalty exclusion clause if decommissioning has not happened over a two-year period.

But such technical arguments are now considered beside the point. For it is a widely accepted truth that entry into government with Sinn Fein without a start to IRA decommissioning would bring Mr Trimble's leadership of Ulster Unionism to a swift and brutal end.

His vulnerability has been the basis of the attitude of both governments over the past six months, as "deadlines" have been allowed to pass. Republicans charge the agreement has been subordinated to the demands of unionism's internal management. And some, certainly in the Irish establishment, fear they may actually have helped the unionists dig the decommissioning trench still deeper.

Small wonder, then, the celebrations of the December 18th "breakthrough" were so muted. For as they clear the way for a showdown on the real crunch issue, both governments are painfully aware that it is in the decommissioning ditch that Mr Trimble firmly stands.