BAGRAM & LONDON: When the Royal Marines from 45 Commando step down from their Hercules transport planes on to Afghan soil this week, their first sight will be a collection of semi-ruined hangars at Bagram, once the largest Soviet air force base in Afghanistan.

Here American troops drink coffee at newly built wooden tables, while others work out on an aging metal bench press nearby. For days British troops have been clearing mines and preparing a site at the base for the 1,700 Marines who will arrive over the next three weeks in the biggest deployment of British troops since the Gulf war. They will double the size of the American-dominated ground combat operations in Afghanistan.

Around the base are the flat Shomali plains, once filled with lush orchards but razed to the ground by fighting between the Taliban and Northern Alliance and still heavily mined. Only in the distance will the new arrivals get their first glimpse of what lies ahead — the jagged, snow-capped peaks of the Hindu Kush. Beyond lies the Pakhtoon heartland, where British troops are mostly likely to go into combat.

Large areas of eastern Afghanistan are still regarded as hostile territory. For more than three weeks US troops have been operating in the mountains at Shah-i-Kot, outside the town of Gardez, in Operation Anaconda, the biggest campaign of the war.

It was the events of Anaconda that led the US to ask for the deployment of the Marines. The move followed questions about US strategy early in the war which, critics say, allowed Taliban and Al Qaeda fighters to retreat and regroup. Indeed, once the Taliban collapsed there was little to stop them retreating into the mountains, from where they have launched a guerrilla campaign.

Many believe that a fear of heavy casualties stopped the generals from deploying ground troops early in the war.

But even in the operations that followed escaping fighters were a recurrent problem.

Officers claim hundreds of enemy fighters were killed in Operation Anaconda. But Afghan commanders told a different story, saying hundreds of Taliban and Al Qaeda fighters were able to slip away. Only a few bodies lie in the bombed ruins of the valleys. In the fighting, especially in the first week of vicious close combat, up to 10 per cent of the US force was injured, with eight servicemen killed.

Unable to encircle the enemy, the Americans could not stop Al Qaeda fighters from slipping away. The British Marines have been sent to ensure that in any future battle there are sufficient troops, and nowhere for Al Qaeda to run.

Likely areas for operations in which the Marines may be involved include that around the town of Khost, close to the Pakistan border, Urgan, a few miles further south, and Ghazni, to the west. It is here Afghan commanders have warned that Taliban and Al Qaeda forces are regrouping and slipping across the border into the Pakistani tribal areas around Miram Shah.

“The things that are going to happen post-Anaconda are more complex,” said Colonel Joe Smith, the US chief of staff of the joint task force.

“We realise the complexities of the culture and of the different regions that we are ready to go into. We are trying to make sure that we understand exactly what is going on as far as the intra-politics are concerned, to make sure we are helping the interim government and not being counter-productive.”

US troops involved in the attack in December on Tora Bora also found that hundreds of enemy fighters had fled to the mountains. British officers say they are aware of the problem and admit that cutting off escape routes will be one of the toughest tasks facing the Marines. It is unclear whether the Marines will risk recruiting Afghans, who have local knowledge but often questionable loyalties.

The Taliban and Al Qaeda fighters know the area well and are better acclimatised to the altitude.

Bagram air base is at 6,000ft, but the Marines can expect to operate in mountains as high as 12,000ft. Stark temperature variations mean the troops will have to carry heavy cold weather gear along with their ammunition. At these altitudes, that will slow their march to a crawl and test their training to the full.

Marines from 45 Commando took part in a training exercise in Oman in November and then went to Scotland for several weeks of winter mountain exercises before leaving for Afghanistan.

The enemy they face is an unconventional force of hardened fighters, most Afghan Taliban with some Pakistanis, Arabs and Chechens loyal to Osama bin Laden. They number in the hundreds, perhaps thousands.

Their weaponry is unsophisticated but it is often deadly. In Operation Anaconda the resistance was unexpectedly heavy. The Taliban and Al Qaeda weapon of choice was a rocket-propelled grenade, which proved devastatingly effective against the US attack helicopters.

Brigadier Roger Lane, commander of 3 Commando Brigade, is due to arrive in Afghanistan as early as tomorrow to lead the Marines. Under agreements, Lane will have the right to exercise a veto on missions suggested for the British.

Within a month, once planning and acclimatisation are completed, the Marines will be involved in the toughest combat the British Army has seen for a decade. “The Marines will start training for about a month when they arrive,” said one British officer. “We expect about two months of combat. We think there are around 2,000 Al Qaeda left. When we are finished, they will all be dead.”

THE EVIDENCE is that Al Qaeda are not just regrouping but are refitting too. US intelligence officials have been analysing equipment found on or around Al Qaeda fighters after the Shah-i- Kot battles two weeks ago.