In the event of success progress should be swift, for a complex new superstructure of administration is in place, ready to roll once the word is given. But failure will bring uncertainty and vacuum; and political vacuum has often been filled with violence.
The decommissioning issue is the only thing now standing in the way of the formation of an unprecedented new form of government. This would be headed by an executive which would include members of David Trimble's Ulster Unionists, of Sinn Fein, of the nationalist SDLP and of the Rev Ian Paisley's DUP.
On the face of it the question is simply one of the destruction of terrorist hardware, but everyone knows that it is much more complicated. At the heart of it lies not paramilitary capacity but generations of mistrust and antagonism.
This can be seen in the fact that most observers, including security experts, regard de-commissioning not as a security issue but a political one. Even a completely disarmed IRA would represent a continuing threat since, if the conditions were right, it could always rise Phoenix-like from the ashes once more, as republican groups have done so many times in the past.
As this period of negotiations opens, the republican community and nationalists as a whole seem as committed as ever to the Good Friday agreement. Well over 90 per cent of nationalists, north and south, voted for the accord in last year's referendums.
Almost all southern nationalists, and probably most northern ones, saw the agreement as a vehicle in which Catholics and Protestants could work together in agreed new institutions. This did not mean that all the Catholics love all the Protestants; many of them dislike unionism as a cause and would see the accord as a way of working against it.
But, whether motivated by the hope of a new beginning or by tribalism, either way they remain united in favour. Where they look divided is in their attitude towards decommissioning, since many nationalists, probably a majority, have a much more flexible attitude than that expressed by Gerry Adams.
There is opinion poll and anecdotal evidence that a great many nationalists think that if de-commissioning is necessary to bring the new institutions into being then it should happen. This is the position of the leaders of all major nationalist groupings with the exceptions of Sinn Fein and the IRA.
Tony Blair's hope must be that Gerry Adams, faced with the choice of movement on decommissioning or the collapse of an agreement so enthusiastically endorsed by Irish nationalism, will opt for the former. But exerting too much pressure on Mr Adams could be a dangerous business, for there have been many warnings that decommissioning could rip the IRA apart.
There are many in the security field who would welcome such a sundering, having for years favoured a "split and crush" approach towards the IRA. This is not, however, Government policy, which is that a unified IRA is preferable to a new collection of small but deadly republican terrorist groups, uninterested in any peace process and dedicated only to the bomb and the bullet.
On the unionist side, some of this has already happened in both the political and paramilitary spheres, where parties and terror groups proliferate. A third or more of Ulster Protestants, at a guess, want nothing to do with any administration which includes Sinn Fein.
Many of those, unsurprisingly, support Ian Paisley, but a fair few are associated with David Trimble's party and thus limit his room for manoeuvre. They include not just voters but also people throughout the party structures, including many of his Westminster MPs.
Over the last year Mr Trimble has shifted away from the Good Friday agreement's ambiguity on decommissioning and has appropriated many of the arguments of the accord's opponents. His position is that he will take no part in an executive before the IRA carries out actual decommissioning. This stance seems to make a fudge or a compromise very difficult.
The logic of the position is that either the republicans will cave in at the last moment; that they will not and stalemate will follow; or that Mr Trimble will execute a public climb-down.
All this is to be viewed against a background of much confusion and much fracturing within unionism, rendering the position there highly unpredictable. This crucial week thus begins with much hope but also with much uncertainty, and no guarantee of success.