The SAS badge depicts King Arthur's Excalibur
It will be, perhaps, the most dangerous mission in a history of perilous engagements, their most famous being carried out in the full glare of television cameras in 1980, when SAS troops stormed the Iranian embassy in London where terrorists had killed a hostage.
The SAS freed the remaining hostages and killed five of the six terrorists, although their spectacular success gave them much unwanted publicity.
The SAS and its motto, Who Dares Wins, was born early in the Second World War, when a British army officer, David Stirling, came up with the idea of a highly-trained special force which might wreak havoc on enemy supply lines, bases and morale.
He joined forces with an Australian, Jock Lewes, an officer in the Welsh Guards who had a talent for improvisation.
Parachutes were the obvious way of getting troops behind enemy lines, but an early practice jump using a scrounged parachute and an unsuitable aircraft put Stirling in hospital for two months.
He used the time to flesh out his plan, and on leaving hospital, he slipped past the guard at High Command headquarters, and managed to convince the top brass that his idea was both necessary and feasible.
The SAS was initially created as a desert raiding force to weaken Rommel's North African logistics network as well as hinder aircraft operations.
Their first successful raid was in December, 1941, when two groups destroyed 61 aircraft at two airfields.
There were to be many more SAS operations in the war against Hitler and later, the SAS saw action in several counter-insurgency operations, in Oman, Aden, Malaya and Borneo.
They were also deployed in Northern Ireland against the IRA and saw intensive action in the Falklands.
During the Gulf War, SAS teams penetrated deep within Iraq to search for mobile Scud missile launchers.
But the experiences of one eight-man team illustrated how the courage and reputation of the elite force could not guarantee success.
After being dropped far behind Iraqi lines, three men were killed and four captured, although they are said to have first killed 250 Iraqis.
A book taking its title from the team's call sign, Bravo Two Zero, made a fortune for its author Andy McNab, a pseudonym of the group's leader.
It has sold 1.5 million copies, been translated into 16 languages, and spawned dozens more books about the SAS, to the consternation of those whose lives could depend on the secrecy of its methods.
The men of the SAS have known hardship before they ever join. The selection process is, as one might expect, gruelling. Probably the most testing element is the Long Drag.
It is a 40-mile trek over the Brecon Beacons, carrying a 55-pound backpack, that does not include the required food and water. Only about one in every 12 soldiers passes the course.
The SAS cap badge depicts not a winged dagger, as was thought, but King Arthur's sword, Excalibur, surrounded by flames.
If British special forces are deployed in Afghanistan to exact retribution for the attacks on the Twin Towers and the Pentagon, they could end up fighting guerrillas they helped to train.
According to a former SAS man, Mujahadeen fighters loyal to America's prime suspect, Osama Bin Laden, were trained by the SAS as part of the West's efforts to bolster resistance to invading Russian forces in the 1980s.
And the British are said to have taught the Afghans how to use deadly Stinger missiles, capable of bringing down helicopter gunships.
SAS troops could be used to guide precision air strikes or attack terrorist camps in Afghanistan. A special SAS unit has apparently been engaged in mountain training in Pakistan for several years.
Some military experts assume their members are already in Afghanistan. Another chapter in their story seems likely to unfold.