Britain is to send its elite mountain warfare troops to Afghanistan to carry out swift raids on the Taliban and Osama bin Laden's al-Qa'ida network in a winter war of attrition.
The official announcement giving details of Britain's entry into the land war will be made today. Between 300 and 500 Royal Marines of 3 Commando Brigade will form the nucleus of the British ground force which will join American forces in the campaign.
Senior politicians and military officers acknowledged last night that the risks of casualties were high in fighting the Afghans on their own terrain. The US Secretary of State, Donald Rumsfeld, yesterday spoke about how tough an opposition the Taliban were proving to be.
The statement in the Commons on the deployment, by Defence minister Adam Ingram, will coincide with Geoff Hoon, The Secretary of State for Defence, briefing commanders in Oman, where the bulk of the British force for Afghanistan is currently on exercise, in Operation Saif Sareea II.
However, most of the Royal Navy fleet will stay behind off Oman, and the full strength of the force is expected to rise to about 1,000 combat troops and up to 3,000 other service personnel in the near future.
The Marines are expected to spend up to three weeks at sea before taking part in land action. That means British involvement will begin after the Afghan winter sets in.
The aircraft carrier HMS Illustrious is expected to remain in the Gulf. The helicopter carrier, HMS Ocean, the navy's largest ship, and the assault ship HMS Fearless will also stay in the Gulf for the time being before returning to Britain to be reconfigured for possible future deployment in the Afghanistan campaign.
Mr Hoon will be holding talks with Sultan Qaboos bin Said in an attempt to allay Omani reservations about ground strikes on the Taliban and their al-Qa'ida allies.
Britain had initially wanted a land base in Oman for the operation, but withdrew the request to due to Omani unhappiness at being used as a launchpad for attacks on another Muslim country. The Marines will, instead, be ship-based using helicopters to carying out logistical shuttle-runs to and from land.
Although the bulk of the 23,000 service personnel involved in Saif Sareea II will start returning to Britain, some are expected to be sent back to join the Afghan force in the future.
The announcement on the British troop deployment was expected earlier this week but disagreements between the Royal Navy and the Army over precisely which units should be used were understood to have caused the delay.
In Washington, Mr Rumsfeld has warned that the US may never capture Mr bin Laden, and that even if it did so, his al-Qa'ida network would remain.
"These are very tough people, they've made careers out of fighting and they're not going to roll over," Mr Rumsfeld said.
"As for bin Laden, it's a big world, there are lots of countries, and he's got a lot of money and a lot of people who support him, and I just don't know whether we'll be successful."
Though Mr Rumsfeld predicted that ultimately Afghanistan's Taliban regime would be overthrown, his remarks are the latest by senior Allied officials acknowledging that the US - like other outside powers which have ventured into Afghanistan before - would have its work cut out to achieve its aims against a battle-hardened enemy who knows the terrain.
Rear Admiral John Stufflebeem, of the US Navy, admitted he was surprised at the "dogged" resistance of the Taliban - who appeared to have stalled if not repulsed a Northern Alliance drive against the strategic town of Mazar-i-Sharif, and who remained entrenched in their lines north of Kabul, despite intense US aerial bombardment. Yesterday US and British planes were reported to have once again pounded targets in the capital, as well as Taliban positions, and guarding the road to Mazar-i-Sharif.
Similar cautions came from Admiral Sir Michael Boyce, the chief of Britain's defence staff, who said that ground troops might have to mount weeks-long operations inside Afghanistan if they were to root out Mr bin Laden and his network.
Admiral Boyce said the Taliban were digging in for a long war. He warned: "We must pace ourselves to sustain a long campaign so that we don't burn out after the first two or three months."
The comments are further evidence that Washington will be unable to meet its goal of wrapping up the most visible part of the campaign before the onset of winter, and the start of the Muslim holy month of Ramadan in mid-November. They are also an effort to prepare US public opinion for a lengthy and messy struggle which may last for years.