"We are faced with a full-blown insurgency," says Pakistani journalist Ahmed Rashid, author of Taliban: Militant Islam, Oil & Fundamentalism in Central Asia.

Four and a half years after they overthrew the Islamic militia that had controlled much of Afghanistan, U.S.-led forces have been forced to ramp up the battle to stabilize this impoverished, shattered country. More than 10,000 U.S., Canadian, British and Afghan government troops are scouring southern and eastern Afghanistan in a campaign called Operation Mountain Thrust.

PHOTOS:Fighting the Taliban in Afghanistan

Even before fighting heated up this spring, Lt. Gen. Michael Maples, director of the U.S. Defense Intelligence Agency, warned Congress that the insurgents "represent a greater threat" to the pro-U.S. government of Afghan President Hamid Karzai "than at any point since late 2001."

More than 500 people �?mostly insurgents �?have died since mid-May in the fiercest fighting since the fall of the Taliban regime. Since Operation Enduring Freedom began in October 2001, more than 300 U.S. troops have died, 165 of them killed in action. NATO's 36-country International Security Assistance Force (ISAF) has lost 60.

Despite the damage they can do, the insurgents do not have enough support to topple Karzai, who was elected two years ago and enjoys international support. "We are not in a situation yet where the Karzai government is threatened," says Joanna Nathan, Afghan analyst for the International Crisis Group, a non-profit research organization. But in places where they are strong, the insurgents have been able to harass government operations and relief efforts �?so much so that reconstruction has come to a virtual standstill in the south and east.

"It is hurting us," says Afghan Finance Minister Anwar ul-Haq Ahady. "We build a school, and they come and they burn it. We build a clinic, and they come and burn it. We build a bridge, and they knock it down. Security is the No. 1 issue."

Fears of new 'training camp'

The fear is that an ungovernable Afghanistan will revert to what it was before the overthrow of the Taliban: a failed state that can spread instability across Central Asia and be used as a launchpad for international terrorism. "If the Taliban get their way, Afghanistan will again become a training camp for terrorists," NATO Secretary General Jaap de Hoop Scheffer told CBC, Canada's public broadcaster, this month.

The influence of the fundamentalist Islamic militia is obvious in Panjwai district, in the heart of Taliban country. Villagers in this dry, dusty plain 15 miles west of Kandahar say they are trapped between the Taliban and the U.S. and Afghan troops hunting them. If they cooperate with the coalition or with the Afghan government, they risk Taliban reprisals.

Just outside Makuan village here, Noor Mohammed, deputized as a security guard at a radio tower, goes to work in plainclothes. "If I wear a uniform, they will kill me," he tells Canadian army Capt. Jonathan Snyder, 24, who is patrolling the area two days after a Canadian convoy was ambushed nearby. Snyder is exasperated: "You shouldn't fear for your life," he tells the frightened man. "They should be fearing for their lives because of you."

The insurgency is a loose alliance of Taliban guerrillas, followers of former prime minister and fundamentalist warlord Gulbuddin Hekmatyar, al-Qaeda terrorists recruited from across the Islamic world, opium traffickers and local fighters whose murky motives are rooted in tribal politics.

Taliban commander Mullah Dadallah told al-Jazeera television last month that the insurgents can call on 12,000 fighters. In an interview, Taliban leader Naseeruddin Haqqani says there also are hundreds of suicide bombers. The Taliban's claims probably are exaggerated, Rashid says, but they can draw on hundreds of fighters.

The insurgency began a few months after U.S.-led forces drove the Taliban out of the Afghan capital, Kabul, in November 2001. It became more effective two years ago, when insurgents switched to new tactics, including breaking up into small groups of 10 fighters or less, attacking "soft" civilian targets and limiting head-on confrontations with coalition and Afghan troops.

Like their counterparts in Iraq, the insurgents use the Internet to pick up tips on making roadside bombs, the U.S. commander in Afghanistan, Lt. Gen. Karl Eikenberry, has said. They increasingly rely on suicide bombers. Writing in The New York Review of Books this month, Rashid noted 40 suicide attacks in the past nine months vs. five in the previous five years.

Franchising terror

Insurgent leaders �?such as Taliban spiritual leader Mullah Mohammed Omar; Jalaluddin Haqqani, father of Naseeruddin Haqqani; and Hekmatyar, who heads the radical Islamic Hizb-i-Islami group �?"do not exert power the way a military general does," Seth Jones, an analyst for the California-based think tank RAND Corp., wrote in the spring edition of the journal Survival. Instead, they leave "tactical and operational" control to local cells, "which act as franchises."

Al-Qaeda, which supports the insurgency with training, supplies and occasionally manpower, operates much the same way.

The loose alliance opposed to the Karzai government and the U.S.-led reconstruction of Afghanistan has gained strength because:

•The insurgents have found sanctuary in Pakistan, "fairly brazenly" staying "beyond the reach of Afghan and international security forces," Nathan says. Pakistan's powerful spy agency, the Inter-Services Intelligence (ISI), supported the Taliban against rival Afghan factions when the fundamentalist movement formed in the mid-1990s. Pakistan's military regime wants to counter the separatist instincts of Pashtun tribesmen who live in both countries. The government's pro-Taliban policy changed under U.S. pressure after the Sept. 11 attacks.

Rashid says Pakistan has done nothing to eliminate Taliban forces operating openly out of Baluchistan, a Pakistani province opposite southern Afghanistan. The reason, he says, is that the Baluchistan insurgents are "pure Taliban" �?remnants of the ISI-supported fundamentalist regime that ruled most of Afghanistan from 1996 to 2001. The insurgents based in Waziristan, by contrast, include many foreign jihadi fighters and members of al-Qaeda �?fighters the United States has pressured Pakistan to pursue. "That suited the Pakistanis quite well," Rashid says.

•Ordinary Afghans won't risk their lives to support Karzai's government, which many view as weak and corrupt. Afghanistan's problem is "not necessarily the strong enemy," Eikenberry said in Washington last month. "It's the very weak institutions of the state."

The government also is widely seen as corrupt and dominated by warlords linked to the bloody civil war during the 1990s. "Day by day, corruption, bribery and narcotics go up," says Noor ul-Haq Ulumi, a member of the Afghan parliament from Kandahar. "Weak governors we have every place. They think only about their benefit, not their country's benefit."

•The United States and its allies have scrimped on money and manpower, critics say. Rashid says Iraq has distracted the United States from the difficult tasks of subduing the Taliban and rebuilding Afghanistan. "For Afghanistan, the results have been too few Western troops, too little money and a lack of coherent strategy," Rashid wrote in The New York Review of Books.

According to RAND, international aid to Afghanistan equals $57 per person, compared with $679 in Bosnia and $206 in Iraq. RAND also found that Afghanistan has one soldier for every 1,000 people vs. seven in Iraq, 19 in Bosnia and 20 in Kosovo. RAND's Jones reckons Afghanistan needs 200,000 Afghan and foreign troops and police officers to establish order. The country has about 120,000.

Insurgents test the resolve of NATO forces in the process of taking over combat responsibility from U.S. forces in southern Afghanistan. The incoming NATO commander, British Lt. Gen. David Richards, insists NATO forces "will deal most robustly" with insurgents.

Rashid says the rules of engagement are "incredibly unclear."

"They bifurcate NATO into countries that will fight and countries that won't fight, and that's a dangerous thing," Rashid says.

The insurgents are eager to bloody the NATO newcomers, to find out which ones will fight and to target those that won't. "This is a testing time, a transition time, and is likely to be messy," Nathan says.

Insurgents "are betting that the West doesn't have the political will to remain in Afghanistan for the long run," Jones wrote. "Proving them wrong is the key challenge."

Sending troops to back Karzai's government and keeping them there is "a sacrifice worth making," Nathan says. "Sept. 11 demonstrated what happened last time the international community abandoned Afghanistan."

Contributing: Zafar M. Sheikh in Islamabad, Pakistan; wire reports

THEY FORM THE INSURGENCY Key players in Afghanistan's insurgency: �?Taliban. Islamic militia known for its strict form of Islam and for harboring al-Qaeda leader Osama bin Laden after the Sept. 11 attacks. Controlled much of Afghanistan from 1996 to late 2001, when forced out by a U.S.-led invasion. �?Al-Qaeda. Group formed by fugitive terrorist bin Laden almost 20 years ago to expand Islam's "holy war." Initially, al-Qaeda evolved to oppose the Soviet occupation of Afghanistan in 1979-89. �?Gulbuddin Hekmatyar. A fundamentalist warlord, leader of the radical Hizb-i-Islami group and former Afghan prime minister. Has been loosely aligned with the Taliban and al-Qaeda against the U.S.-backed government of President Hamid Karzai. �?Jalaluddin Haqqani. Senior Taliban commander believed to be operating on the Pakistan-Afghanistan border. �?Opium traffickers. Profiteers who want the Afghan state to remain weak and unable to shut down their operations. They often ally with anti-government Taliban forces. �?Tribes. Afghanistan's 11 major tribes compete for control of various regions. Some have worked with Haqqani to oppose U.S. troops. Many tribes living near the Afghan-Pakistani border do not recognize the boundary. Insurgents have attacked government targets in Afghanistan and escaped into Pakistan. Sources:  Wire reports, Council on Foreign Relations, CIA World Factbook , Library of Congress, Power and Interest News Report