This Afghan war is young; it may get a lot messier

'The al-Qa'ida fighters have many advantages. They know the terrain and have numerous escape routes'

Recovering drunks are told never to forget the "yet" factor. It is an injunction to resist the smugness induced by meeting somebody worse off. For example, a sober drunk meets a man who has lost his family or has been to prison, and says: "Thank God that hasn't happened to me." The wise counsellor will whisper in his ear that little word "yet". It may not have happened but it may yet come to pass.

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Recovering drunks are told never to forget the "yet" factor. It is an injunction to resist the smugness induced by meeting somebody worse off. For example, a sober drunk meets a man who has lost his family or has been to prison, and says: "Thank God that hasn't happened to me." The wise counsellor will whisper in his ear that little word "yet". It may not have happened but it may yet come to pass.

I thought about the "yet" factor this week when Geoff Hoon announced he was sending more than a thousand Royal Marines to the mountains of Afghanistan. The suddenness of the announcement unleashed a torrent of nervous speculation with no end of Vietnam comparisons. All of a sudden the ghosts of Dien Bien Phu, Khe Sanh and Hue were summoned from history's ashes. "Wasn't it the US Marines that Lyndon Johnson sent into Vietnam first?" a colleague asked.

Well yes it was, but in circumstances rather different from those currently prevailing in Afghanistan. The allied operation in Afghanistan is not Vietnam. Not yet. But there are similarities that any political and military leader would do well to consider. The foreigners and their Afghan allies hold the towns, but even here, as in Afghanistan, much of the power is in the hands of powerful local figures. The central government is weak and riven with discontent, though, unlike South Vietnam, the leader is a man of probity.

The guerrillas roam across a difficult terrain, sucking in ever larger numbers of troops and then vanishing. Tonnes of bombs fall from a great height but, like the airstrikes in Vietnam, they cannot destroy the enemy. Unlike Vietnam, the guerrillas do not have the backing of a conventional military force, but they are being helped by elements of Pakistan's intelligence services.

They live in tunnels and caves that allow them to pop out and rain fire on advancing troops. I read of an American soldier who expressed shock at these "guys who come out and shoot at us and then vanish". He shouldn't just study the Vietcong but go back to the British military archive of the 19th century Afghan wars. Change the language slightly and you hear the words of a British infantryman fighting his way through the Khyber under the rifles of the Pathans. But it isn't Vietnam. Yet.

I plucked Neil Sheehan's great book A Bright Shining Lie off the shelf the other night, remembering how it described the beginning of the war in Vietnam. The first account of a major battle between the American-backed South Vietnamese regime and the Vietcong would make uncomfortable reading for General Tommy Franks. The battle of Ap Bac erupted in the Mekong Delta in late 1962 when 350 Vietcong tied down an army four times their number, backed by jet aircraft and artillery and with helicopter support.

As Sheehan writes: "They suffered 18 killed and 39 wounded, light casualties considering that the Americans and their Vietnamese protégés subjected them to thousands of rifle and machine gun bullets, the blast and shrapnel of 600 artillery shells and the napalm bombs and assorted other ordnance of 13 warplanes and five Huey gunships. With the weapons they held in their hands the guerrillas killed or wounded roughly four of their enemies for every man they lost." I'm not saying the battles of the last few weeks in Afghanistan are of precisely the same order; but only a fool would refuse to admit there are worrying similarities.

Most of those who refuse to see the similarities are not fools. They are literate, intelligent people. Many of them write perfectly sensible columns, until they come grinding along to the "war on terror". They are the ones who crowed about the swift victory in Afghanistan, turning a blind eye to hundreds of years of history and the difficult questions about who would rule who and for how long. For these muddle-headed optimists any questioning of the Allied strategy was synonymous with being soft on terror.

I think it was Matthew Parris who, predicting the war might end in tears, offered to eat his hat if proven wrong. Hold on to your hat, Mr Parris. This war is young and may get much messier. Suppose we set aside the Vietnam experience on the grounds that it is too different to allow for valid comparison. Consider the experience of the Soviets. Mark Urban wrote a detailed study of the Soviet attempts to crush the Afghan resistance during the 1980s. As a former Army officer, Mr Urban brought a keen tactical mind to his subject. His accounts of the endless Soviet attempts to crush the mujahedin in the Panshir valley makes for cautionary reading.

True the technology available to attacking forces has improved dramatically over the past decade, and the élite forces deployed by the Allies are better trained and equipped than their Soviet predecessors. But the Taliban and al-Qa'ida fighters have many advantages. They know the terrain and they have numerous escape routes along the porous border with Pakistan. They have a vast constituency of support in the tribal areas on the border and the renegades within Pakistan intelligence provide them with information and matériel.

They also have a great deal of time. Winters and summers will pass and the guerrillas will wait and wait. They don't have to do very much. An ambush here and there, and they can tie down huge numbers of troops.

Modern armies operate by tight schedules. They have to because they are driven by massive political pressure. The tyranny of the quick fix is no less problematic for military planners than it is for the rest of us. George Bush may have pledged an endless war against terror. But I doubt the American people will thank him if their sons are still dying in Afghanistan in 10 years. That is not an improbable scenario; the Taliban and the assorted crazies who make up al-Qa'ida love nothing more than endless war. They don't face the pressure of public opinion and press scrutiny. They just kill those who disagree.

Which is why the Allied side needs to be a great deal less presumptive about public opinion. Mr Bush is still in a strong position. The memory of the twin towers attack is vivid and has, for understandable reasons, created an environment in which no significant figure in the US has the courage to question the prosecution of the war. Not yet.

Things are much less sure for the British Government. The mass of people do want the Taliban and al-Qa'ida destroyed, but they want to know how it's going to be done and need to be kept informed. Waking up and hearing on the radio that 1,700 Royal Marines are on their way to Afghanistan is too much of a surprise, even for those who support the aims of the war.

We are told, rightly, that Afghanistan is a dangerous environment in which Britain can expect casualties. But it is time the Allied leadership looked at the objective factors operating against them in Afghanistan and then spelt these out to the public. There is the potential for a long and bloody conflict, and if our leaders know this, they must pass on the news. Support for the war remains relatively strong, and there is no mass movement against our deepening involvement. Yet.

The writer is a BBC Special Correspondent