It is a measure of the frustration, or desperation, of the Irish and British governments that the chairman of the international arms decommissioning agency, Gen. John de Chastelain, has been called upon to break the political impasse.

His entry into the negotiations was flagged by Bertie Ahern in the Dail on Tuesday when he said the Belfast Agreement could not succeed on the basis that there would be no decommissioning.

"It will only work, "the Taoiseach said, "if the institutions are established now and if confidence is created that a process of decommissioning will be achieved under the aegis of Gen. de Chastelain and in the context of the implementation of the overall settlement." During the past six months, Mr Ahern has shifted ground and emphasis on a number of occasions in the face of the obduracy of the IRA and the intransigence of the Ulster Unionist Party.

Before Easter, the most intense pressure was being applied to Sinn Fein and the IRA. In February, Mr Ahern declared: "I am on record in recent weeks and months as saying it is not compatible with being part of an executive unless there is at least a commencement of decommissioning."

The approach led to the Hillsborough negotiations, the terms of which were rejected by the IRA during the Easter holidays. That package envisaged weapons being put beyond use, as "an obligation" on the IRA. In return, a shadow executive would be formed and North/South institutions activated.

The Taoiseach was later to comment that while the Hillsborough proposals had departed from the letter of the Belfast Agreement, which did not specify prior decommissioning, it was a reasonable approach.

Having failed to convince the IRA, however, political pressure swung back on David Trimble and the Ulster Unionists.

The formula that emerged from intensive discussions at Downing Street envisaged the establishment of an executive in advance of arms decommissioning and no direct reference was made to an "obligation" on the IRA to put weapons beyond use.

That approach was rejected in Belfast by the Ulster Unionists even before David Trimble returned home and his leadership has remained under intense pressure ever since.

The Downing Street formula has been peddled in slightly different forms since then, as attitudes hardened within both communities arising from the approach of Drumcree and an upsurge in sectarian violence, punishment beatings and killings.

At the heart of both the Hillsborough and Downing Street formulae was the role of Gen. de Chastelain.

For while politicians argued bitterly over whether Sinn Fein could be appointed to, or participate in, an executive in advance of decommissioning, the larger issue of the IRA's commitment to arms decommissioning remained unexplored.

True, Sinn Fein signed up to the Belfast Agreement which provides for the decommissioning of all paramilitary weapons by April 2000. But, within three weeks of that event, the IRA announced it would never disarm. And it repeated that assertion some months later, causing Mr Ahern to declare there could be no such thing as an armed peace.

Last May, the Ulster Unionists suggested that formal recognition by the IRA of its obligation to decommission arms by next April could unlock the door to political progress. But the overture was met with a deafening silence.

Gen. de Chastelain has now been asked by the governments to investigate and pronounce on the matter in advance of next Wednesday's deadline.

The general has written to representatives of all paramilitary organisations checking their commitment to the decommissioning deadline; asking whether firm undertakings could be given in that regard and what modalities were envisaged in bringing it about.

His report, due to be delivered to the governments next Monday, could make or break this phase of the process.

The Canadian has always been regarded as an immensely valuable asset by the governments: a man trusted by the hard men within both communities, whose word or judgment on decommissioning could be used to facilitate political movement.

But his usefulness was seen as finite: a trump card that could be played once or, at best, twice. And the governments were holding off engaging him until they were confident of a positive outcome. Now, it would seem, his time in the spotlight has come as Tony Blair goes all out to secure the establishment of a Northern executive and the implementation of decommissioning.

These make-or-break developments were not of Bertie Ahern's making. A solo run by the British Prime Minister lumbered the two governments with a deadline of June 30th. And the Taoiseach went along with it reluctantly.

From the British point of view, June 30th was an obvious choice. On that date, power will be devolved from Westminster to a new parliament in Scotland and to an assembly in Wales. And Northern Ireland institutions were seen as part of that major constitutional reform programme.

But such linear thinking made no allowance for the convoluted politics and sectarian divisions of the North. And it ignored the determination of the various parties not to be forced into taking unpalatable decisions.

Already, a government deadline for the triggering of the d'Hondt process and the establishment of a shadow executive has come and gone, lost in the campaign for the European elections.

Mr Ahern and Mr Blair will travel to Belfast tomorrow to meet the various parties and to kick-start this new phase of the process.

They will assess the situation during the weekend and return to the North on Monday to receive Gen de Chastelain's report. A decision by the Parades Commission on the Drumcree march is also due at that stage.

Deadlines can help to concentrate minds. But they can also threaten a fragile equilibrium. The task now facing Mr Ahern and Mr Blair is to get the Ulster Unionists and Sinn Fein to "jump together".