So it is to be a moment of truth for Mr David Trimble - and for Mr Tony Blair. The Ulster Unionist leader responded prudently and cautiously to the statement from Mr Gerry Adams and Mr Martin McGuinness: "If there are going to be talks they will be on the basis that the consent of the people of Northern Ireland will be paramount in determining their destiny."
Pressed to say if he would be a participant in those talks, Mr Trimble advised Channel Four's Jon Snow: "Just be patient."
Yet on Thursday Mr Trimble told Mr Blair that unionist patience was at an end. His leadership, he told the Prime Minister, would be destroyed if he entered negotiations involving Sinn Fein without a clear commitment to a process of parallel disarmament.
The British-Irish proposals for removing decommissioning as a bar to negotiations were simply not acceptable. And the long wait for confirmation of a renewed IRA ceasefire will have left time aplenty for rueful reflection.
Already Mr Trimble could hear the angry voices - not just from the DUP but from within his own ranks - suggesting that he might be guilty of the "crime" laid at the door of his predecessor, James Molyneaux.
Mr Trimble will deny that he misread the intentions of Mr Blair and the new British government. Friends of the UUP leader insist, to the contrary, that Mr Blair only recently appears to have changed position.
But even that defence contains confirmation that their own assessment has not been sustained by events.
The truth is that the UUP leadership had convinced itself Mr Blair shared their belief that there was no question of a ceasefire on terms sufficient to gain Sinn Fein's entry to talks. Within a short time of the election, the private suggestion was that Mr Trimble would enjoy a better relationship with Mr Blair than he had with Mr Major.
The conviction was that Mr Blair (they seldom seemed much concerned with Dr Mowlam's intentions) was in the business of moving the process ahead without the republican movement. The confident talk was of early movement toward the creation of a new Northern Ireland assembly and a looser, altogether ill-defined, arrangement involving Dublin in a "British Isles" context.
It is in this context alone that Mr Trimble's perceived "flexibility" must be seen. Of course he was prepared to "park" the decommissioning issue - because, to his mind, it was entirely academic. And those who this week have noted a "toughening" of his position have simply missed the point.
What the UUP leadership appeared to miss was the build-up of solid evidence that Mr Blair and Dr Mowlam were prepared to bend over backwards to secure a ceasefire and an "inclusive" process.
The UUP gave Dr Mowlam the benefit of the doubt when, during the general election, she appeared to move toward a fixed point of entry to talks consequent upon a ceasefire. Ditto when on the BBC's Newsnight on Wednesday, May 21st, she sent what may come to be seen as the most important of all London's signals to Sinn Fein and the IRA.
Asked if - in the event of a ceasefire acceptable to her - the talks would proceed if the unionists walked out, Dr Mowlam at first hedged, resisting such negative and hypothetical scenarios.
But when pressed that she had said the opposite - that the settlement train could leave the station without Sinn Fein - Dr Mowlam replied: "And it will leave the train [station] without unionists if they don't . . . I mean that is the state of play."
It seems safe to assume that Mr Adams and Mr McGuinness have reached their decision on the understanding that that remains the state of play.
And it does not seem unduly cynical to suggest that in the past two days the British government and Sinn Fein have effectively combined to maximise Mr Trimble's difficulty.
It appears it was only under pressure from Mr Trimble and Mr Ken Maginnis that the British made their limited, and piecemeal, disclosures about their "clarification" process with Sinn Fein.
The Sinn Fein letters to Mr Blair have not been made public. But the British response, together with the published aide memoire and Mr Blair's Commons statement of June 25th, amount to a package which effectively meets the requirements of the Hume/Adams proposals rejected by Mr Major last autumn.
Sinn Fein last night bought a British promise that it would enter the talks process six weeks after a ceasefire; that ministerial contacts would open immediately upon the cessation; that talks with the Independent Chairman would swiftly follow; that substantive negotiations will start on September 15th; that decommissioning will not be allowed to block those negotiations or provide a pretext for Sinn Fein's subsequent exclusion; and that the British are serious about a timetable which envisages a settlement in place by next May.
By announcing for a ceasefire yesterday, republicans will consider themselves to have symbolically removed the "unionist veto" over the process. By publishing terms to Sinn Fein on Thursday, the British denied themselves any obvious scope to make further significant concessions to Mr Trimble.
The most serious, and least enviable, onus now falls on him. For weeks Mr Blair has been telling Sinn Fein the settlement train is ready to leave without them. For all their denials - and unless Mr Blair, like Mr Major, is to be accused of "bad faith" negotiation - it seems implicit in the British scheme that they are prepared to move without Mr Trimble on board.
"New Labour" has started much, on a broad front, in a hurry. And there is an inevitable question about the wisdom of the declared timetable, and the implications - for stability in Northern Ireland - in pursuit of a destination opposed by mainstream unionism.
But Mr Trimble must calculate that many in "Old" Labour would cheer Mr Blair to the rafters in any confrontation with him. He must also know the perils, from his own perspective, of vacating the ground to the control of the two governments - and of permitting unionists to be cast as the enemies of peace.
In truth, the UUP never wanted or intended to find itself in negotiations with Sinn Fein. Decommissioning - the invention of the previous British government - enabled them to avoid that result. But Mr Blair has removed that device for immobility.
At the same time he has placed his personal interpretation on the principle of consent - offending republicans with his assertion that it presupposes a partitionist settlement, and Northern Ireland's continued place within the United Kingdom.
For Mr Trimble, sitting down with Sinn Fein would undoubtedly be painful. He might cheer himself with the thought that his discomfort will be nothing to that of republicans brought, under pressure of London and Dublin, to an acceptance of that fundamental political reality!