Bill Clinton was right about Northern Ireland. The question is not whether the peace will be challenged - no doubt it will - but how the North responds when the challenge comes.
The President made the point in Belfast, to the surprise of many; and, after he and Tony Blair met Omagh's brave survivors, he repeated it at the gathering for peace in Armagh.
His was not just a stirring message, it recognised the reality of life - and death - in Northern Ireland. And there were bad reasons for the cautionary note.
No doubt he remembered that, when last he spoke to the people of Belfast, almost everyone was heartened by what he had to say and by the way it was received.
But, even as he switched on the lights on that December evening, members of the Provisional IRA were preparing to start the New Year with a bomb.
Their explosion at Canary Wharf, a characteristic response to British procrastination, killed off a ceasefire, on which great hope had been raised, and the lives of two Londoners who got in the way.
Two-and-a-half years later, the Belfast Agreement has been negotiated, signed and endorsed by the electorates, North and South. There is an Assembly, there will be an executive, the work of building a better-balanced society has begun.
But if the Provisionals' bomb provided a bitter epilogue to Mr Clinton's visit in 1995, the "Real IRA"s' attack on Omagh underlined the need for political solidarity on this occasion.
So David Trimble and Seamus Mallon stood together on the steps at Stormont, a building designed to assert inequality, now commonly associated with partnership and change. Together, Trimble and Mallon greeted Clinton and Blair.
The Prime Minister's addresses were as encouraging as the President's - and no less challenging. Both say the future is now in Northern hands. The people have had their say. Their leaders must follow.
The President bluntly reminded them of their commitments: decommissioning; the formation of an executive; changes in policing; an end, once and for all, to punishment beatings; the release of prisoners, and the introduction of measures to strengthen human rights and promote equality.
Mr Blair's tactical advice was equally pointed: they must be prepared to lead, whatever the criticism and whoever the critics - even when they were politically close to the leaders themselves.
For most, it will mean breaking the habit of a lifetime. Politicians on every side have found the temptation to move at the pace of the slowest all but irresistible.
Mr Blair seemed to speak directly to David Trimble and Gerry Adams. And, indeed, for all the eloquence and power of President and Prime Minister, it was clear that Mr Trimble's was the speech to watch.
He counted among welcome developments of late what he called the beginning of the reconstruction of those who had been engaged in violence. (Mr Adams and his party were little more than an arm's length away.)
Though he didn't list them, there was no doubt about the developments the First Minister-designate and leader of the Ulster Unionist Party had in mind.
The IRA had promised to help discover the bodies of people its members had murdered; Mr Adams had said "The violence we have seen must be for all of us now a thing of the past, over, done with and gone;" and Martin McGuinness had been appointed Sinn Fein's representative to the international body on decommissioning.
The IRA's refusal to let people know where their murdered relatives lay was one of its most callous decisions; its unwillingness to change was both inexplicable and unforgivable.
Mr Adams's statement followed precisely the wording of a demand made by Mr Blair. But there was a difference:
Mr Adams is in a position to announce the war is over (and didn't); Mr Blair is not.
Mr McGuinness's appointment may yet prove to be the most significant of the developments noted by Mr Clinton and Mr Blair and welcomed by Mr Trimble.
It acknowledges the party's paramilitary links (as the loyalists of the PUP and UDP have done) and should put an end to the distracting fiction that Sinn Fein and the IRA are not related.
Contrary to the views expressed by IRA leaders - in recent statements and in their current presumption to police the "Real IRA" - it offers hope of change.
At least it suggests a willingness to take part in the project steered by John de Chastelain which, as the text makes clear, is an essential part of the Belfast agreement.
By the same token, it brings a step closer the prospect of simultaneous progress in politics and security, which is how the authors of the agreement seem to have planned its implementation.
It may even help Mr Trimble, who faces the most serious internal challenges from opponents of the agreement, his own would-be rivals and those who simply don't share his view that people can change.
On Thursday he said: "If you take the road of peace and do so in genuine good faith, you will find in me a willing leader . . . Every move you make towards peace I welcome. Every pledge you make to peace - I will hold you to it."
The first test of leadership on all sides will take place on Monday when the leaders of the parties in the Assembly meet under Mr Trimble's chairmanship.
This is likely to be followed by a meeting attended by Mr Trimble, Mr Mallon and Mr Adams. An event which may prove more fruitful than the handshake for the cameras that some had expected this week.
Those who talk about politics as choreography have had their share of music hall this week - and it isn't over yet. But there have been achievements for which Mr Clinton, Mr Blair and Bertie Ahern deserve credit.
Mr Clinton was right about Northern Ireland. As Mr Blair said of him: "No President of the US has done more."
But when he began to talk about Northern Ireland as an example - to the Israelis and Palestinians, to the Albanians and Serbs or to the Indians and Pakistanis - he invited other, more questionable comparisons.
Governments not only have the right to defend their people, they are obliged to defend them and the states they are elected to govern.
The Irish and British governments have responded to this obligation - in our case against groups presuming to speak for us and murder others in our name.
Americans, too, have been murdered by terrorists, in Nairobi and Dar es Salaam. Mr Clinton, too, feels obliged to respond; many Americans consider vengeance to be their right, as their attitude to the death penalty shows.
The admiration we feel for Mr Clinton's opposition to terrorism in Northern Ireland or in east Africa should not blind us to the vengeful bombing of Afghanistan and Sudan.
George Bush believed the United States to be the world's policeman. He was wrong. And Mr Clinton is wrong if he feels obliged to follow his wretched example.