Dominic Lawson: The only options are to double up in Afghanistan or leave

At a risk of sounding callous, the number of casualties is actually small for a war

On the first Friday of 2002 – 4 January, to be exact – the Foreign Minister of the new US-backed Afghan government, Dr Abdullah Abdullah, announced that the world's most famous one-eyed Mullah, the Taliban leader Mohammed Omar, was "under siege and surrounded". Dr Abdullah's boss, Hamid Karzai, confidently added that Mullah Omar would be "delivered to the US" to stand trial – presumably on the basis that he had given hospitality and security to Osama bin Laden and the other al-Qa'ida leaders behind the assault on the World Trade Centre.

As it turned out, Mullah Omar had not been completely surrounded. He managed to flee Helmand on a motorbike, and found refuge somewhere in Pakistan. Where he is now, neither the Afghan government – nor probably the Pakistan government – know. All they do know is that Mullah Omar is still the leader of the Taliban, and still commanding its operations against the American troops – and ours – in Afghanistan.

If, eight years ago at the outset of Operation Enduring Freedom, anyone had predicted such an outcome, he would have been ridiculed – just as it might have seemed incredible when the Soviet Union invaded Afghanistan in 1979, that 10 years later they would leave in despair, having lost 14,000 soldiers in combat, with many more terribly injured.

We make predictions about Afghanistan at our peril, therefore; and anyone who says with certainty what will happen in that benighted region if the international force withdraws – or if we stay – should be treated with about as much respect as we would accord a fairground Tarot reader. To use the language of the former US Defence Secretary, Donald Rumsfeld, the best we have are "known unknowns".

So when Lord Ashdown writes, as he did last week, that "withdrawal would mean the certain fall of Pakistan", we should not regard this as anything but a reasonably educated guess – and certainly worthy of no more credibility than the prediction that if US forces abandoned Vietnam, the whole of South-east Asia would fall to Communism like a pack of dominos.

Although strategists like to divide the world up into zones of influence, the truth is that most conflicts are intensely local in origin. It is especially true of Afghanistan, which is not really a nation at all, but a mess of competing tribal rivalries. This is presumably what the senior US official in Afghanistan, Matthew Hoh, meant when he declared "I fail to see the value in continuing US casualties and expenditure of resources in what is really a 35-year civil war" – and quit.

If we are to pull out our forces, however, it should not be for the wrong reasons. While arguing for staying, Lord Ashdown was writing in the immediate aftermath of the killing of five British soldiers by an Afghan police officer they were training. He portentously declared that this incident had "fractured a central plank of the only strategy we have". Many other commentators took a similar line, seeing this incident as an example of the uncontrollable treachery of the Afghans we have been training to enforce law and order.

Yet, according to an investigative report by the Sunday Times' Christina Lamb – who has been covering Pakistan and Afghanistan for the best part of 20 years – this incident was no plot against the Brits. It turns out that the killer was a young officer, called Gulbuddin, who had been serially sexually abused by a much more senior officer (these things happen in Afghanistan); he swore revenge against his abuser, and targeted him with a machine gun while the senior officer was in the company of a number of British soldiers.

According to an eyewitness report: "The five British soldiers were killed simply because they were present and considered to be the officer's protectors." So if we are going to have a debate about pulling our forces out of Afghanistan, let it not be because of some entirely random act of sexual revenge by a deranged young man.

It is true that those deaths brought the tally of British soldiers killed in active operations in Afghanistan to over 200; and for every one of those we need to multiply by the number of close family members bereaved to get a sense of the scale of personal grief caused. At the risk of sounding callous, however, the fact is that the number of fatalities is remarkably small for a war – and it is a war – that has lasted for eight years.

The same would be true for the Americans, who have, over the same period, lost just over 600 soldiers, with about 4,400 wounded. Moreover, this is not like Vietnam, where thousands of conscripts lost their lives; the US forces in Afghanistan, like ours, are career soldiers, volunteers all.

However, the continuation of such casualties is tolerable, both to the armed forces and to the nation as a whole, only if they are borne in a campaign which has a definable purpose and an end in sight – preferably the end which was defined at the outset. In this country Gordon Brown has been abject in defining its purpose. He gives the impression that it is purely to safeguard Britons from al-Qa'ida threats which would otherwise be launched from Afghanistan. If that is the basis of our argument, we should have invaded Pakistan and put troops on the streets of Bradford.

When General McChrystal asked President Obama for an extra 40,000 troops in Afghanistan, he did not do so just to limit al-Qa'ida attacks on New York City – it is because he is attempting to defeat the Taliban. Like most military men he is not a counter-terrorism expert – he is trying to win battles against a rival force. What the US army is engaged in is counter-insurgency, not counter-terrorism, although admittedly the distinction between the two can get blurred.

It does worry me – it should worry all of us – that the British Government is committing our troops to a joint operation with the Americans, if the objectives being pursued are not the same: that is only a recipe for confusion. For this is an international operation, led by the Americans, but supported – in blood and treasure – by many other nations apart from ours.

The debate in this country about whether or not "we" should withdraw is conducted as if this was an autonomous British military exercise in Afghanistan, as it was in the 19th century. This is a ludicrously anachronistic and parochial way of looking at it. It is a UN-mandated international mission, although obviously the US has the dominant role.

President Obama has now dallied and fretted for almost three months without giving a response to General McChrystal's plea for an extra 40,000 men. The reason for his hesitation is obvious: none of his civilian advisers wants to make that commitment. Obama, however, is commander-in-chief, and will probably find a compromise number of extra troops, fewer than the military believes it requires, but not so small as to appear to be letting the soldiers down. It will be the worst of all outcomes, since it will satisfy no one, and risk greater casualties without giving the force required for even the hope of a conclusive military victory.

I am no military man, but I suspect there are only two rational approaches at this stage: either a massive additional force from the Americans, bigger even than McChrystal has called for – or rapid withdrawal, with all the loss of face that that would entail. A halfway house would be political cowardice; and a poor reward for the immense courage of our soldiers.

d.lawson@independent.co.uk