Just occasionally the House of Commons rises to an occasion. Yesterday it did so in the most paradoxical of ways, by treating what may prove to be the biggest breakthrough yet seen in the Northern Ireland peace process, with a sombre, almost deadening, caution.
It was entirely right to do so. Northern Ireland has been on an emotional white-knuckle ride between hope and fear for too long for any other kind of reaction to be appropriate. David Trimble, not naturally the most eloquent of men, has to sell a new form of words from the IRA to a meeting of his still restive party on 20 May. Dissident republicans, anxious to derail the often faltering, but curiously stubborn, journey from the gun to democratic politics, remain a potent force, and could become more so. Even if devolution returns soon - an outcome which seemed to many out of the question a mere two months ago - there will be many, perhaps heartbreaking, setbacks.
And yet there was just a sense, also appropriate, that Peter Mandelson's Commons statement signified that something pretty momentous had happened in Belfast last weekend. Which it surely did. The big advance over the formulation issued by the IRA in February is not so much that it makes explicit a willingness by the IRA to put arms "beyond use", something that the unpublished February statement, although in hedged terms, also did.
It is rather, first, that the IRA's bottom line has been made fully public, while in February the republican leadership was unwilling to do that, on the grounds that the rank and file had not been fully consulted. As a result, David Trimble does not have to rely, to anything like the same level, on nods and winks obliquely reflected in a gnomic report from General John De Chastelain.
Second, the language of the new statement, while not wholly free of classic Provo-speak, is much less opaque than the first, which could easily have been, on a hostile but plausible interpretation, seen as linking any movement on arms to the final removal of the British presence in Northern Ireland. But finally, and most importantly, it contains the entirely new commitment to allow the inspection of arms dumps by third parties. Which means that the IRA is meeting the demand that the Unionists had long made for an early gesture, or confidence-building measure, to show good faith - a "massive" move, in the words of one ex-republican yesterday.
As a result, it is difficult to see the outcome other than as a vindication of the decision to suspend the institutions on 11 February. That decision, for which Peter Mandelson largely took the flak (though Tony Blair was also intimately involved in it), was opposed by both the Irish, and at least initially, by the White House. It allowed the republicans, and indeed the Irish government, to depict Mandelson as being biased towards the Unionists; it was probably one of the reasons why the Irish government insisted, at least initially, on dealing directly with Tony Blair's office, in particular his energetic and trusted emissary Jonathan Powell. The fact remains that the post-suspension formulation of the Provisionals is a good deal more robust and substantive than the one that immediately preceded the collapse of the institutions. Maybe it wasn't surprising that John Major, the first British architect of the peace process, should have said that the advance proved that suspension was right. But it was more so that the former Labour Northern Ireland spokesman, Kevin McNamara, gracefully conceded it had "confounded" his pessimism.
This is only partly because it disproves the myth that the republicans would never yield to pressure. It also afforded both governments some space; having shown the Unionists that it was prepared to do what prominent republicans had publicly declared it would never do, the British government is now in a stronger position than it was to persuade the Unionists that the IRA have now gone as far as they are going to go.
This has two consequences. One is the - compared with the much bigger picture in Northern Ireland - minor one of Mr Mandelson's own career. If the settlement sticks, then on one level it is easier to bring Mr Mandelson back to London. Having discharged his obligations in Belfast, it could be argued, he could come back with honour to fulfil some shadowy portfolio which would allow him to join Gordon Brown in running the election campaign more or less full time. But while precedent suggests that this could change quite rapidly, that isn't apparently the current thinking in Downing Street.
It appears that Mr Brown would prefer Mr Mandelson not to be given a ministerial role in London before the election. The Chancellor is almost certainly right. There will be plenty to do in Belfast, and in any case it would be easier for Mr Mandelson, relieved of many of his day-to-day responsibilities, to play his part in general election planning while remaining in office. He would also be proving that he can do his job effectively not only when it is glamorous but also when it is a little less so.
Fascinating as this may be in London, the much greater consequences are for Northern Ireland Unionism. Of course there are problems for them, not least their continuing pain at the changes to policing. Strenuous attempts are being made by British and Irish officials to find a formula to defuse the problem posed for Mr Trimble by the Ulster Unionist Council having rejected any future deal without further concessions on this highly symbolic issue.
But this cannot alter the fact - which all David Trimble's body language yesterday suggested he understood - that if Unionism fails to rise to this occasion it may not get another like it for a generation. There are unanswered questions. What about the dumps that the IRA will not expose to view? Will there be tangible evidence that the long-term process of "deactivating" - the new John Major-coined vogue term - weaponry is fully under way by July next year? And no doubt Unionists will have to make an act of faith, though hardly a reckless one, to trust that if the republicans default, first General De Chastelain and then a joint review of the agreement by Dublin and London will point the finger of blame.
Nevertheless, the tentative signs are that, while there will be a fight with the hard-line oppositionists, the basically pro-agreement Unionists are now returning to the Trimble fold. And rightly so. Unionism should not forget that the principle that Northern Ireland can only change with a consent of its majority is fully secure. Or that the Republic no longer claims sovereignty over it.
To say that a Unionist rejection of this offer of a lasting return to politics might finally push an exasperated British government into some menacing Anglo-Irish agreement foreshadowing joint sovereignty may be going too far.
But if the Ulster Unionists fail to rise to the occasion, there is no doubt who, this time, will get the blame.