Tony Blair is watching his greatest political achievement dwindle away

In Northern Ireland at Easter 1998, Mr Blair had momentum; he had the big Mo. But he let little Mo squander it

It would be a delicious irony, were it not so serious. At the end of last week, in the north-east of England, Tony Blair called for a big conversation. As he was doing so, he got one, in the north-east of Ireland. The English version was merely choreographed fraudulence; If Mr Blair really thinks that anyone will mistake that interchange of treacly banalities for a conversation, he is losing his political touch.

In Ulster, however, there was no treacle. It was a real conversation, albeit a brief one. It consisted of one word of two letters, beginning with "N". The word was reinforced by a two-fingered gesture. As a result, Mr Blair's greatest achievement is in trouble. So are the people of Northern Ireland.

It need not have been like this. Indeed, there have been several occasions over the past 35 years when it was not inevitable that Ulster would take the wrong turning. Each time, the ultimate responsibility lay with a failure of intellect and will at Westminster. The same will be true if the Good Friday agreement eventually collapses.

That agreement is worth revisiting, for it was Tony Blair's finest hour. Those tempted to write him off as a mere tawdry thespian, whose gimmicks are becoming increasingly cynical, as is his audience, should first cast their minds back to the fraught final hours before Good Friday, 1998. An agreement was more or less in being, but there was a problem. Very few of the leading figures would agree to it. Then Tony Blair went to Belfast and took command. This was not choreography: Mr Blair seized those faltering negotiations by the scruff of the neck and rescued them. With the sole exception of Ian Paisley, who did not matter so much in those days, the PM imposed his diplomatic, intellectual and moral authority on all the principals, from the Irish government to Sinn Fein.

It might seem odd that he had to. However difficult it may have been to arrive at the Belfast agreement, it was the only one which could ever have worked. It was intended to embody four principles. All sides should accept the democratic legitimacy of Northern Ireland. All paramilitary organisations should disarm and disband, in exchange for an amnesty and the right to take part in politics. Both the main cultural traditions in the province should be treated properly. The electoral arrangements for devolved government would ensure that the Catholic minority had representatives at the Cabinet table.

That is the only viable basis for stability in the province. Yet the very difficulty of persuading its politicians should have been a warning. The basis for an agreement in an Israeli-Palestinian peace deal is equally obvious. When ethnic or religious conflict is involved, "obvious" should not be conflated with "easy to achieve".

But after Good Friday, Tony Blair was master of events. The agreement almost gave him a title to greatness. What a tragedy it was that he let everything dribble away to littleness, because he would not exert his grip. Back in 1968, General Sir Ian Freeland, the first senior soldier sent out to try to control the troubles, made the wisest comment by anyone charged with administering Northern Ireland in the province's history. He said that by imposing calm, the Army had created a window of opportunity. The politicians must now use that to the uttermost, or it would rapidly close. They did not. It did.

Mr Blair has also allowed his window to slam in his face. At Easter 1998, he had momentum; he had the big Mo. But he let little Mo squander it. All the British officials involved knew that Mo Mowlam combined limited ability with an erratic personality. Her judgement could be relied on; it was almost always bad. Yet she was left in charge of the implementation.

As a result, there was none, in several crucial areas. Under Good Friday, there was no obligation to release a single paramilitary prisoner unless and until his organisation had made substantial progress with decommissioning. But Mo Mowlam seemed happy to empty the jails, as long as she got a hug from Gerry Adams.

A further difficulty emerged. The final text of the agreement had been drafted by exhausted men in the small hours. As the weeks passed and the only pressure from London came in the tight squeeze of Dr Mowlam's embraces, the IRA grew emboldened to examine the small print and to argue that it was not obliged to do what everyone assumed that it would have to do. In response, the British government displayed irresolution. The spirit of Good Friday was easily summarised: no guns or else no government. The IRA began to hope that there could be a third way under which it would stay in Cabinet while retaining much of its weaponry, all its paramilitary structures and an intimidatory control over its own areas.

The result was predictable: Unionist outrage. In the early Seventies, Martin McGuinness had been wanted for terrorist outrages. Few Unionists believed that the charges were unfounded. Yet here he was, Minister of Education, while the IRA had not decommissioned and the authorities were making no attempt to prevent the paramilitaries running significant areas of working class Belfast. It is not just anti-Catholicism - after all, some of the paramilitaries are Protestant - which leads Unionists to object to the geographical entrenchment of a criminal sub-culture on a scale which had not hitherto been seen in modern Europe, north of Naples and Sicily.

When all this was accompanied by moves to hollow out Unionist culture by eliminating royal associations and insignia from public buildings, and especially by renaming the RUC, there was an inevitable response. The Belfast agreement depended on the political assertiveness of a strong moderate Unionism. That could not survive repeated snubs and disillusion. In the early days, those who had drafted the agreement were happy to own up to "constructive ambiguity". But as it became clear that the agreement's implementation would not eliminate the IRA's military capability, increasing numbers of Unionists were equally happy to translate constructive ambiguity into a plain Ulster idiom: "We have been lied to."

It is not clear why Tony Blair allowed his achievement to slip away. There was certainly an element of timidity; he was not prepared to move Dr Mowlam and risk her resignation at the height of her popularity in the country and in the Labour Party. There was also an element of Mr Blair's other besetting weakness: narcissism. It was as if he expected everyone in Ulster to remain dazzled by his political genius; he could not understand the need for continuing hard graft.

Perhaps he now does, but it may be too late. Though Mr Blair is now assuring us of his willingness to listen to the British people, we can be certain that there is one person to whom he does not want to listen. Yet he will have to do so, often. He now faces the disagreeable task of taking heed of the Rev Dr Ian Paisley.