Whatever course U.S. military action takes against Osama bin Laden and his Al Qaeda terrorist organization, if the United States goes into Afghanistan, a significant role is likely to be played by forces already on the ground, a loose grouping known as the Northern Alliance.

Members of this alliance, also known as the United Front, ruled Afghanistan before the Taliban took control of the desolate, mountainous and largely impoverished nation in 1996. The Northern Alliance has been fighting the Taliban since then.

The Northern Alliance's military strength has been estimated as high as 50,000, but more realistic assessments by Jane's World Armies put the number at 20,000 or less. In small groups, its forces are active all over the country and are in firm control of all or portions of six provinces in the northeast, about 10 percent of Afghanistan.

This territory includes old Soviet airfields that could be used as forward bases by U.S. aircraft or commando units. The Northern Alliance is eager to help the American effort, analysts say. It already has been receiving military equipment from Russia, via Moscow's close ally Tajikistan. According to American sources who have maintained contact with the Northern Alliance, the U.S. and Russia are arranging to provide the Afghan rebels with more.

The apparent hope is that, in concert with U.S. aerial assaults and possible covert and special operations force actions, the Northern Alliance might serve as the lever to topple the Taliban regime and make way for the installation of a more moderate and less anti-American government in Kabul.

But at best, the Northern Alliance is a military wild card.

"I would say their strength is more like 6,000 in terms of serious combatants," said Anthony Cordesman, director of Middle East studies for Washington's Center for Strategic and International Studies. "But much more could be mobilized and more will appear if there's an attack on a specific area."

British Royal Army Maj. Charles Heyman, editor of Jane's World Armies, agreed.

"There are all sorts of clans who will join for a bit and then move off again and go cold on them, and then come back in the fold," he said.

Barnett Rubin, a leading Afghan expert said that troop strength is not the real issue. "These are not disciplined forces that are going to march out and fight each other according to their battle order," said Rubin, director of New York University's Center on International Cooperation. "How many forces each side has depends on how people feel the wind is blowing."

On paper, the Taliban has a field army of 45,000 men, an air force of about 20 old Soviet Su-22 "Fitter" ground-attack planes, MiG-21 "Fishbed" fighters and a few Mi-35 "Hind" attack helicopters.

Taliban armor amounts to about 100 aging Soviet T-54 and T-55 tanks. It also has 200 armored fighting vehicles and armored personnel carriers, plus artillery pieces.

According to Jane's, the Northern Alliance has about 30 old Russian tanks, and perhaps 50 armored fighting vehicles.

"It's been really, really difficult to get spare parts for these old tanks," Heyman said. "They have to cannibalize other tanks, and only a handful are still operational. The U.S. will have to almost certainly provide them with spare parts and things like that to give them a chance against the Taliban."

Most of the Northern Alliance's armored force has been engaging Taliban troops in northeast Afghanistan's Panjshir Valley, not far from the capital of Kabul and one of the few pieces of terrain in the country conducive to tank operations.

"Tanks are almost a liability," Heyman said. "They make good mobile fire support bases and nothing else. The chance of one tank being knocked out on a mountain road means you can't get the other tanks past. It's ideal ambush country, so you're really talking about boots on the ground."

U.S.-made missiles

Both sides also are equipped with U.S.-made shoulder-fired Stinger missiles acquired when Washington supplied military aid to Afghan insurgents in their successful 1980s war against the Soviet Union. The ground-to-air missiles inflicted a heavy toll on Soviet helicopter gunships, but mostly in daytime attacks. Sophisticated U.S. helicopters like the Apache would be expected to operate at night.

The Northern Alliance has been intensifying military efforts in recent days, according to news reports from the region, but this loose grouping of forces remains a big political question mark.