The single condition involved was acceptance by the British and Irish governments of material drawn up in a series of meetings between the Sinn Fein president, Gerry Adams, and John Hume, leader of the Social Democratic and Labour Party. The Hume-Adams agreement, which has never been published, included a mechanism setting out how Sinn Fein could enter the political processes in the event of an IRA cessation.
Dublin, propelled by public opinion, pressed London to join with it in drawing up a statement which might be acceptable to the IRA. The result was December's Downing Street Declaration, in which the two governments set out their respective positions.
The Declaration, which went through up to 20 drafts, was a separate entity from Hume-Adams but it covered the same areas and it had the same purpose: to set out the positions of the two governments in a way which did not betray democratic principles, but which also stood a chance of persuading the IRA to abandon terrorism.
The ministers and officials who drew up the declaration and hammered out its final wording produced their own distinctive document, but they did not do so in a vacuum. They did their drafting with the Hume-Adams agreement sitting on their desks as a constant reminder of what the IRA would settle for.
Some contend there is only a whisker of difference between Hume-Adams and the Declaration, while others say the differences are vital. But the differences were enough for the IRA army council to give the order not to dump arms, but to send mortars raining down on Heathrow airport last week.
Now come the questions. Was the IRA serious in the first place, or was it all a con? Was all the talk of peace nothing more than a cynical device to make short-term propaganda gains? Or did Gerry Adams and his associates fail to persuade the republican movement to abandon violence? And will he keep trying? The problem with attempting to answer these questions is that the republican leadership is, as one observer put it, a black box.
A couple of dozen leaders debate their moves and make their decisions through 'democratic centralism' behind very tightly closed doors.
The political line emerges in the form of faxes to newspaper offices. Some bear the name of Gerry Adams, some of Tom Hartley, some of Martin McGuinness; but the style is generally identical, as though drawn up by a committee. The terrorist line becomes clear only when the guns and bombs actually go off.
It could hardly be otherwise: conflicting political signals would immediately be seized on as evidence of splits; and leaks about terrorist intentions could have meant the Heathrow mortar bombers finding themselves staring down SAS gun barrels.
This endemic secrecy, coupled with the fact that few politicians actually meet Adams and his associates, means that judgements on their intentions must be based on fairly meagre data. None the less, much of the political activity of the last six months has been based on a judgement that Adams is serious about peace.
The first to assert this was John Hume, who took considerable political risks in promoting that idea: he was, after all, deliberately raising the stock of his great rival for the leadership of northern nationalism. Next came Albert Reynolds and the Irish government, who overturned the practice of decades to communicate directly with Sinn Fein.
The tack taken by Hume and Reynolds has been warmly endorsed by the Catholic Church, through Cardinal Cahal Daly. In other words, all the major pillars of constitutional nationalism have publicly indicated that they believe Adams is a potential peacemaker.
So too, in a less open way, has the British government. Before embarking on the Declaration, teams of military, security and political analysts made careful assessments of Adams's character, his intentions and the extent of his control over the movement.
Several weeks ago, a high- level government source summed up: 'Some pretty careful assessments were made before we took things to where they are now. There is confidence that Adams is serious, but he doesn't yet speak for the whole movement by any means.
'The odds seem to be slightly tipped against him being able to deliver the republican movement in the short- term. But the movement can't ignore the Declaration and maybe at some stage, if Adams could deliver Tyrone, Fermanagh and south Armagh, or felt he could marginalise them, then we might be taking things forward.'
Another government source reported that opinions differed within departments but added: 'The consensus seems to be that if he could carry them he would go for it, but he's not prepared to do it unless he's certain that they'd nearly all fall in behind him. My guess is that his main preoccupation is to maintain the unity of the movement.'
In Dublin, the Heathrow attacks produced despondency and near-despair: to most it seemed that the light at the end of the tunnel had just been snuffed out. But there was also visible a tiny little hope, perhaps stemming from little more than wishful thinking, that something might yet be salvaged from the wreckage.
Operationally, the two governments have some weeks or months to take stock. The Downing Street Declaration represented an offer to the republicans, but it does not depend on them for its validity. While waiting for the republican response, London and Dublin have not been easing up on security co-operation or holding back on moves towards inter-party talks.
The recent hard line taken by the Unionist parties has, in any event, put paid to the prospect that round-table talks could be revived in the near future. Local political energies are being devoted not to finding agreement but to fighting June's European elections.
July ushers in the marching season, which means no talks are conceivable before the autumn. By then it will be clear whether the IRA is intent on rebuilding its campaign to its previous levels.
By doing so, it would be opting for a return to the cold. The propaganda gains made by Adams in the United States came because he was perceived as a peacemaker, but the Heathrow attacks and his careless use of the word 'spectacular' have severely dented that image.
Those gains could quickly be reversed: 'What goes up must come down,' as one observer put it. In the coming weeks efforts will be made to see whether some spark of life remains in what the republicans themselves christened the peace process.
All will be watching to see whether the idea of peace was just a chimera, or whether something other than mortars will emerge from the black box.