British forces know the justice of their cause, but not the aim of the battle

The force of 200 British commandos readied for action in Afghanistan is rather smaller than the "powerful force" we were led to expect. Which may be just as well, because the prospects for a land invasion of the country that has seen off British forces three times in history are not good.

The force of 200 British commandos readied for action in Afghanistan is rather smaller than the "powerful force" we were led to expect. Which may be just as well, because the prospects for a land invasion of the country that has seen off British forces three times in history are not good.

Nevertheless, yesterday's announcement was an historic event. Britain has now committed its forces to fight side by side with US troops on the ground in Afghanistan, in a conflict likely to last for three or four years, according to Admiral Sir Michael Boyce, chief of the defence staff. It seems that when it comes to risking lives on the ground, the international coalition against terrorism consists only of the US and Britain. Other countries – Germany, Canada, Australia and Japan – support the US unequivocally, and there are reports that French special forces might already be operating in Afghanistan, but they are not the core of the Partnership of Nations.

It is not surprising if our isolation makes some Britons uneasy. There is no virtue in standing shoulder to shoulder with the US just for the sake of it, and indeed there is some cost to such a stance in that it renders Britain more of a target. Hence the appearance of "Bush and Blair" as the infidel doublet of al-Qa'ida propaganda since 11 September. The question that should be asked, however, is not why others are not with us, but whether we are right – and if it is right for British forces to take part.

Certainly, there are justified reservations. It would have been better if Tony Blair had presented himself more as a spokesman for the whole of Europe rather than as a Gladstonian or even Gaullist advocate of his nation state's unique role in world affairs. It would have been better if the bombing had been less continuous and if cluster bombs had not been used.

It is necessary to use military force on the ground against al-Qa'ida, however, because there is no other way to bring Osama bin Laden's organisation to justice. That means, inevitably, going to war against the Taliban, because the regime protects and supports the terrorists.

The problem is that the purpose of any action in which British commandos may be deployed then becomes blurred. Their aim may be to hunt down al-Qa'ida terrorists or to destabilise the Taliban regime. In both objectives, Donald Rumsfeld, the US Defence Secretary, has sounded an uncertain note.

It cannot inspire much confidence among the Britons on HMS Fearless when Mr Rumsfeld first says how very difficult it will be to find Mr bin Laden before being forced to "clarify the semantics" by saying: "I think we're going to get him."

Nor was it the best response to last weekend's raid on an empty barn in southern Afghanistan or to the Northern Alliance's failure to make much progress towards Mazar-i-Sharif, for Mr Rumsfeld and US service chiefs to express surprise at the toughness of the Taliban forces.

When British forces do go into Afghanistan for more than a trial run, they deserve clear objectives worth fighting for. Mr Blair tried to remind his audience on British Forces radio yesterday of why they are engaged in Afghanistan, but it will take more than emotional talk about messages on the answering machines of loved ones in the US on 11 September to make the case.

The US and British forces know why they are there. What they may not know is precisely how al-Qa'ida will be "destroyed" and how the Taliban will be "removed and disabled", in the Prime Minister's words. Perhaps there is a secret master plan which cannot be disclosed until it is put into action. But more public clarity of objectives would help to convince us that military action is not only justified, but likely to succeed.