SO IT is to be a "moment of truth" for Sinn Fein, and, too, for Mr David Trimble. The Ulster Unionist leader, like Sinn Fein and much of the Irish political establishment, has been astounded by the speed of Mr Tony Blair's engagement with the Northern Ireland question.

Initially, at least, Mr Trimble appeared to relish the promised "smack of firm government" from London. His approval of Mr Blair's defining speech in Belfast on May 16th was equal to Dublin's (well-concealed) disapproval. Mr Trimble liked (and likes) the obvious linkage in the Prime Minister's mind between Labour's devolution proposals for Scotland and Wales and his altitude toward the creation of a new Northern Ireland Assembly.

The UUP leader (elected, after all, to be more cunning and less flexible than James Molyneaux) can hardly have imagined Mr Blair an instant convert to the cause of an "internal" Northern settlement.

Both before and since the election, Dr Mo Mowlam has explained Labour's abandonment of "Irish unity by consent" by a steely and consistent assertion that the Downing Street Declaration and the Joint Framework Documents settle the context in which a solution has to be found.

Unionists' hopes have appeared high, for all that. The assumption seems to have been that Mr Blair (if not necessarily Dr Mowlam) shared their assessment that the IRA would not restore a ceasefire on terms sufficient to gain Sinn Fein's entry into talks. The consequent expectation was that Mr Blair would contrive to push the process forward without republican involvement.

That may well in the end be the result of the Prime Minister's "last chance" offer to Sinn Fein to climb aboard his so-called "settlement train". However, the uncomfortable reality for Mr Trimble this morning is that, equally, it may not.

This afternoon in the House of Commons Mr Blair will inform MPs of the steps his government has taken, and proposes, in a bid to secure a restored IRA ceasefire and an "inclusive" talks process.

Much of the expected detail has been well rehearsed in this newspaper and elsewhere in the past week. It will be attended by a demonstration of the Prime Minister's anger at the IRA murders of two RUC officers in Lurgan, a "doubly cynical" act which Mr Blair believes was perpetrated by people determined to thwart his efforts to create the conditions in which to seek a lasting settlement.

Nor will there be anything contrived about Mr Blair's cold fury. Sources close to him say his anger is all too real and the IRA has fundamentally misjudged him if it thinks to proceed by way of a "twin track" strategy combining continuing violence with the offer of peace.

The Prime Minister, it seems, is in deadly earnest when he tells the republican movement that its moment of decision has come. It is no more than a statement of the reality that, if the republicans refuse to rejoin the political process, both governments will have to proceed without them.

But crucially, for all his anger at the Lurgan killings and his ingrained distrust of the IRA, Mr Blair is not set to resile from the offer outlined to Sinn Fein in that aide-memoire of Friday, June 13th. In addition to" that, and the minutes of the "clarificatory" talks between his officials and Sinn Fein, the British government will publish its proposals, agreed with Dublin, to remove the "decommissioning" hurdle to substantive political negotiations.

Those proposals, detailed elsewhere in today's Irish Times, mark the final death of the so-called "Washington 3" requirement for prior decommissioning of paramilitary weapons. They will easily, and fairly, be characterised as another Anglo-Irish fudge. But the most important thing is that this fudge appears non-negotiable.

At their meeting with Mr Blair yesterday, the Ulster Unionists pressed for clarification. They hope to find it in Mr Blair's Commons statement. But they know the Prime Minister cannot alter the substance of the document he has agreed with Mr Bruton.

The decommissioning deal pre-emptively announced by Mr Bruton represents the all-important, final piece in the jigsaw carefully assembled by London and Dublin over the past few weeks.

With Dublin backing, it would seem to permit Mr Blair to say he has met every republican requirement for the restoration of an unequivocal ceasefire.

Together with the June 13th proposals, it opens the way for Sinn Fein's entry to the Stormont talks after a period of some six weeks; provides for decommissioning to be addressed alongside substantive negotiations to be under way in September, on the basis of the Mitchell Report; and establishes a time-frame for the negotiation process itself, with May 1998 the target date for completion and agreement endorsed by referendum.

Mr Trimble fears the envisaged process could take him to next spring without any actual decommissioning of weapons occurring. Indeed, his first objection on sight of the proposals was that they required no commitment by paramilitaries to any decommissioning.

He detests what he sees as the linkage between decommissioning discussions and other "confidence-building measures", for which he reads "political concessions". And "he is likely to press Mr Blair for assurances that they "share a determination that Mitchell means "actual decommissioning" is expected to occur in parallel with the proposed negotiations, and that the proposed Independent Commission, with its verification powers, will be established well in advance of September.

Even given such assurances, it is accepted in London and Dublin that Mr Trimble might well walk. Regardless even of the DUP, that may well be the instinct of many in his own party faced with the prospect of sitting down with Sinn Fein within a matter of weeks. One senior MP last night said he feared Mr Trimble would be unable "to swing this within our own constituency".

AGAINST that, other unionists know, or at least suspect, some in Dublin and in the SDLP would be sanguine at the prospect. If the Provos reject the deal, Mr Trimble gets what he wants, a reduced process operating to a much more limited agenda. If Mr Trimble rejects the deal, he will be cast as a man with no interest in an inclusive process, deemed incapable of doing serious political business and, almost certainly, will find himself with no process at all.

The dangers for the Ulster Unionist leader can hardly be overstated, given the worsening realities on the ground in Northern Ireland. But as he ponders them, London will be encouraging him to consider the risks which the republican leadership is being invited to run: a commitment to the disarmament of all paramilitary organisations, an end to punishment beatings, a renunciation of force as a means of political influence, and agreement to abide by the outcome of negotiations which, as Mr Blair said on May 16th, point to a partitionist settlement rooted in the principle of consent.

From a traditional republican perspective, Mr Trimble's discomfort must seem mild compared with that of Mr Gerry Adams and Mr Martin McGuinness.