George Mitchell's involvement in Northern Ireland, which began in early 1995, was supposed to last for just four months. He was to be an economic envoy, concentrating on trade and investment matters.

George Mitchell's involvement in Northern Ireland, which began in early 1995, was supposed to last for just four months. He was to be an economic envoy, concentrating on trade and investment matters.

No one is quite sure whether President Bill Clinton, when he appointed him, thought of it as a short-term deal, or whether he had a sneaking suspicion that it would go on for so much longer and assume such crucial importance.

The former senator now flies out of Northern Ireland with a lot more grey hairs than he had when he flew in. Some stem from the ageing process, but others are doubtless due to the frustration, exasperation and near-despair he has so often known in Belfast.

In public he has been the cool calm centre of events, ready to flash his reassuring smile at the cameras, and equipped with apparently infinite patience. The book he wrote about his time in Belfast reveals, however, that like everyone else he has more than once felt like giving up.

He was called in at the end of this summer after the repeated failure of government to square the circle on decommissioning and devolution. His review was to be a short sharp exercise, but it dragged on for nearly three months.

In chairing the talks that led to last year's Good Friday Agreement, the senator was required to shepherd eight different parties towards accord. In that part of the process David Trimble and Gerry Adams never spoke to each other. This time the task was both more specific and more difficult, in that he was primarily bringing the two men together.

In his memoirs, Mr Mitchell noted: "At the heart of all the problems in Northern Ireland is mistrust. Each disbelieves the other. Each assumes the worst about the other."

So it proved this time round: he said yesterday that early exchanges in the talks had been "harsh and filled with recrimination".

But under his calming influence things settled down and some sort of relationship was established. His skills were invaluable, as he brought to bear his experience as an army intelligence officer, lawyer, federal judge and majority leader in the US Senate.

The Unionist and republican leaders were well aware they needed to do business together; where Mr Mitchell came in was in providing a safe context in which they could try things out on each other without giving hostages to fortune. In addition to his own skills, Mr Mitchell's presence was a continuing symbol of United States, and indeed wider international interest in the Northern Ireland problem, and a reminder too that the world was growing impatient for progress to be made. He therefore served as a source both of pressure and of lubrication.

He departs from Northern Ireland garlanded with congratulations and gratitude, having helped to transform the atmosphere from one of doom and gloom to one of hope and indeed optimism.

Yet although he has helped the peace process over a number of huge obstacles, everyone knows that many more hurdles lie ahead. There will certainly be more crises down the road and when they come the call may yet go out to George Mitchell to return on yet another rescue mission.