In one of the earth's most inhospitable parts, it requires a special kind of warfare to fight in Afghanistan's rugged mountain terrain. While a ground mission to Afghanistan may not be the first element of a US response to last week's terrorist attacks, it seems inevitable it will come to that.

The US and Britain could deploy a substantial, well-prepared force. But it will be a risky task in a barren region with a reputation as a military death-trap.

Landlocked Afghanistan, bigger than France, is 80 per cent mountains, with extremes of climate. Two-thirds of the country supports little or no vegetation. It is littered with debris from the decade of Soviet occupation which ended in withdrawal in 1989. About 15,000 of their soldiers were killed.

The British, too, found Afghan marauders and tribal warriors too much for them. Their retreat from Kabul in 1842, in the first Anglo-Afghan war, marked a low point in army history: a column of almost 17,000 was wiped out by knives, musket balls and frostbite. Unlike either of those precedents, experts foresee a short, specific "in and out" operation. It would not attempt to control the whole territory, nor imitate the Soviets' tactics. But one expert warned: "It's no easy task."

In cruel terrain, the expedition would face relatively nimble, acclimatised and ruthless opponents. The challenge lies less in the firepower of the Taliban forces - used Soviet armour and old US Stinger missiles - than in their determination.

"You need high-calibre, specially selected professionals, not newly-trained troops," said a former British commander. He suggested the force might be at maximum division- strength - about 10,000 - but was likely to be smaller. Equipped with light artillery, it would be flown in by helicopter, probably after a wave of cruise missile attacks.

It would need to be backed up by combat aircraft using Pakistan airspace. "Nobody would do this without massive air superiority," he said.

Apart from their secretive special forces, the US and Britain have units that co-operate closely in mountain training. Unlike France - one of the other Nato allies with significant capability in the field - they have similar operating procedures.

The US has a dedicated light infantry division, the 10th Mountain Division, attached to the 18th Airborne Corps and used in the Balkans, Haiti and Somalia. The British Royal Marines commando brigade, about 5,000 strong, now on exercise in Oman, is also trained in mountain warfare.

Mobility is the biggest problem. The force must have support from helicopters, which would be operating in hazardous conditions. While aircraft would be flown from carriers or Gulf bases, helicopters would need to operate from closer in - Pakistan, a central Asian republic or even a forward base with fuel dumps inside Afghanistan.

A nearby base would also be needed to evacuate wounded or fly in heavier equipment if necessary.

Difficulties include navigation, vulnerability in tight valleys, and physical endurance. Troops would carry up to two days' supplies of water, food and ammunition, as well as clothing and equipment. Problems increase with snow, which might come from late October.

Their opponents would be less encumbered, using stashes of supplies, and familiar with the terrain.

The risk for any such force is becoming encircled and needing to extricate itself. But one expert commented: "In Afghanistan you're not looking at a major threat of being surrounded by a sophisticated enemy. The real danger, which the Russians found, is that these people resist remorselessly."

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