Three reasons for Britain's commandos to move in, do their job and get out fast

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Leading article: We need to be told what British troops will be doing in Afghanistan

The announcement that 1,700 Royal Marines and Army commandos were going to fight – not keep the peace – in Afghanistan came as a surprise to many. The people had been told before Christmas, by Tony Blair, no less, that the war in Afghanistan was almost over. But, in the past week or so, something has changed.

The main changes have been the worsening violence in the Middle East and the realisation by the United States that pouring more petrol on the Middle East fire with an attack on Iraq would be unwise. Second, a week ago Osama bin-Laden's half-brother, Sheikh Ahmad, had a phone call saying he was alive and well. If the motive for American and British troops to find him had flagged, this news must have encouraged a renewed effort to finish the job.

Third, it is spring. Unconverted and uncaptured Taliban and al-Qa'ida fighters who went to ground for winter will pop up again. But the spring also makes it easier for the US and British, with their overwhelming air power and intelligence assets, to find them.

The choice of a 1,700-strong battle group based on 45 Commando Royal Marines (about 750 men) plus commando-trained Army artillery, engineers and logisticians makes sense. It is a specialised force designed and trained to operate in Arctic and mountain conditions. No plan survives contact with the enemy, as the great German General von Moltke said, but the aim is probably to deploy them for three months with up to a month of fighting. As the Taliban and al-Qa'ida fighters become weaker and more tired, fresh troops can make a huge difference.

The surviving Taliban and al-Qa'ida members are believed to be operating in groups as small as two or as big as 300. The old military rule of thumb is that three-to-one superiority is required for an assault, so the total force of 1,700, 45 Commando plus all the other components of a balanced force, looks just right.

The British gain advantages from deploying this force now. Their spell as leaders of the International Security Assistance Force (Isaf), to which they have deployed 1,500 troops, is coming to an end. Their peace-keepers in Kabul are to be replaced by Germans and the overall leadership of the force will pass to Turkey. With our other responsibilities reduced, this is a good time to switch to war on the ground.

The Americans can still be scathing about peace-keepers, so, by deploying a full-scale force with the United States, the British will gain valuable leverage and influence over US actions. They will also gain intelligence from captured prisoners that America might not have passed on. Britain may even be able to influence how those prisoners are treated.

But the British commandos will stay no longer than necessary. If there is one rule in Afghanistan, it is do the business – fast – and get out.

The author is professor of military science and doctrine at Cranfield University