The question of whether to talk to opponents while they are killing your soldiers and other people is one of the most sensitive in the area of conflict, full of difficulties and dangers.
In Northern Ireland the judgement of most of those involved is that dialogue played an important part in sustaining a peace process which has produced today's coalition government. But it was not an easy task. And the Belfast experience showed that the regrettable lesson is that peace comes not at the start of a peace process but towards the end of it.
Violence continues as the combatants edge towards the table, and may even increase as they seek to strengthen their negotiating positions. This unfortunate fact is another reason the idea of dialogue raises such difficult questions.
For most of the Belfast Troubles the notion of talking to the IRA and similar groups was publicly regarded as anathema, with the idea of "talking to the men of violence" routinely ruled out.
But it emerged recently that even when the most combative language was being deployed – for instance during the Thatcher era – secret back-channels existed for surreptitious communication.
Republican leader Martin McGuinness, for example, was involved in direct talks with a senior British intelligence officer who sounded him out on the idea of an IRA ceasefire, to be followed by high-level talks.
Many detailed documents were exchanged between London and the IRA as the two sides weighed up each other's motives. The exercise continued even as IRA bombs killed civilians and caused huge damage in the City of London.
At one point, the IRA used violence to make a political point, when it fired mortar bombs at Heathrow airport. This was regarded as the end of the peace process until forensic experts reported that the devices had been designed not to explode. The attack was, in other words, meant to show what the IRA could do if the process broke down.
Many elements in the political, military and intelligence spheres disapproved of suggestions of having contact with those they had spent decades fighting. The argument was strongly made that talking might only encourage the gunmen and bombers. But the key calculation within officialdom was that the IRA and its political wing Sinn Fein were in serious internal debate on continuing their "war".
Such a decision requires much analysis and even mind-reading, for the enemy's thinking is never clear. Is the enemy united enough to deliver peace, or are there too many disparate groupings? Are there hawks and doves in there? If so, would opening talks strengthen the less militant factions?
Most of all, a judgement has to be made on whether all or most of the enemy would, now or later, contemplate a negotiated settlement. If so, the key point is to promote and cultivate those who would settle for less than complete victory.
David Miliband, in saying yesterday that Afghanistan's problems are "not susceptible to a military solution" was signalling that the Government is not intent on outright defeat of the Taliban.
His hope is to encourage the more thoughtful in the Taliban ranks to conclude that they too cannot achieve a military solution. While he will also hope that this might lead to an early breakthrough, old Northern Ireland hands within the Foreign Office will think of this as planting seeds which can take years to mature.
One of the key concepts in the Irish peace process was an explicit offer to republicans that if they agreed to abandon violence they would not be regarded as vanquished foes but would be offered a place in new political structures.
This worked in Belfast, so that Martin McGuinness is today no longer regarded as a menace to the state, and now serves as a senior government minister.
Kabul is very different from Crossmaglen but the proposition that worked in Northern Ireland is on offer in Helmand Province: that those who are today part of the problem may eventually become part of the solution.