Moving only at night, lying low and keeping their eyes open during the day, small groups of American and British commandos are already scouting the ground in Afghanistan.
But they're looking for traditional military targets, not Osama bin Laden, according to former special forces soldiers familiar with the sort of operation now under way against suspected terrorist bases in Afghanistan.
The commandos are likely doing "standoff surveillance and observation, slow work," says Mike 'Blue' Thomas, a former soldier in the British Special Air Service (SAS). "They are simply doing information- and intelligence-gathering, covert target reconnaissance," adds Col. Phil Wilkinson, another former British special forces commando, who now works at the Centre for Defence Studies at King's College, London.
"The ideal, if they turned up trumps, would be to find Osama bin Laden, but that is highly unlikely," Colonel Wilkinson says.
Locating Mr. bin Laden, say British and American veterans of special operations and covert action, is a job for spies, not military forces who stay out of sight and make no contact with local people behind enemy lines.
US officials have confirmed that special operations teams have been inserted into Afghanistan, where bin Laden, the prime suspect in the Sept. 11 attacks on the Pentagon and the World Trade Center, enjoys the protection of the ruling Taliban authorities.
The vanguard troops are believed to have been drawn from the top-secret US Delta Force and the elite British SAS. Both units sent men behind Iraqi lines during the Gulf War, in search of mobile Scud missile launchers.
SAS and Delta soldiers work in teams as small as four men, and can operate independently and secretly for up to two weeks at a time without being resupplied, so long as they can find water, according to men who have carried out such operations.
They are trained to seek out and identify potential targets for air strikes or other assault, and then to observe them for as long as it takes to evaluate the targets for operational planners at headquarters. They can then act as forward air controllers, guiding strike aircraft to the targets by radio instructions and laser markers.
In Afghanistan, the teams are likely to be seeking out military training camps, satellite communications bases, and "anything occupied that provides a human target", says Col. Wilkinson.
US officials have said that in the opening strikes of their war against terrorism they will not distinguish between terrorist bases and targets belonging to governments that harbor terrorists.
The special forces seeking out such targets wear military uniform, but sometimes disguise themselves as well with cloaks, local headgear, and greasepaint to darken their skins.
The basic rule, however, is to stay out of sight.
"Contact advertises the fact that you are there," says Mr. Thomas, who fought with the SAS for more than 20 years in Oman, the Falklands, and Iraq, among other places."It is instilled in everyone from square one not to get involved with the locals at all," once they are behind enemy lines.
In Iraq, the teams worked on their own in the desert, though they would sometimes tag their Landrovers onto the end of Iraqi military convoys to avoid detection as they traversed enemy terrain, according to SAS veterans of the 1991 Gulf War.
In Afghanistan, some of the teams are likely to seek assistance from anti-Taliban soldiers fighting with the Northern Alliance, which controls between 5 and 10 percent of the country's territory.
British special forces troops covertly trained the Alliance, led by Ahmed Shah Masood until his assassination last month, for several years up to 1996, according to British officials.
The CIA is also reported to have worked with Mr. Masood more recently, seeking his help to kill bin Laden.
The Northern Alliance guerrillas could be counted on to provide food from supplies they have cached throughout the mountains in the areas where they fight, and to offer guides.
This, says Wilkinson, poses "the major question right now: Who can you trust?
"This is a part of the world with shifting allegiances," he points out. "Operational pressure means that our soldiers will be patrolling with the local folk, whose trustworthiness cannot be guaranteed."
Even if the Northern Alliance guerrillas do prove reliable, the chances that the special forces now on the ground might find bin Laden seem remote.
For years in the 1960s and 1970s, Thomas recalled, SAS troops fighting alongside the Sultan of Oman's forces against Marxist guerrillas sought out the rebel leader, Said Hoff.
"It was just like bin Laden today," says Thomas, who lived for months at a time in the mountainous desert of southern Oman, which is very similar to the terrain in Afghanistan.
"We chased him round for years, but he was like a phantom. Our intelligence was always two or three days old. It is very, very difficult to catch someone like that unless you have someone on the other side helping you."
"To catch bin Laden in Afghanistan, you are going to need human intelligence from inside his network, and that seems unlikely," adds Wilkinson.
If bin Laden is located, however, says Wilkinson, "it could be a special forces operation that will eventually apprehend him."