A complex network of insurgents is battling U.S. forces along the Afghanistan-Pakistan border. The groups include:

_Afghan Taliban: A hard-line Pakistani-sponsored movement that began forming during the 1970s as part of the mujahadeen, or freedom fighters, who battled the Soviet Union's occupation of Afghanistan until the Russians were ousted in 1989. The Taliban emerged as a united entity in 1994 and took control of the country in 1996, instituting a strict and often brutal sharia, or Islamic law.

Led by Mullah Omar, the Afghan Taliban sheltered Osama bin Laden in the years leading up to the attacks of Sept. 11, 2001. But the group was toppled shortly after the U.S. and allied invasion a month later. The U.S. government estimates there are roughly 25,000 members of the Afghan Taliban. Mullah Omar remains a fugitive.

_Pakistan Taliban: Formed more recently, the group is known as the Tehrik-e-Taliban Pakistan, or Pakistani Taliban Movement. It initially was led by Baitullah Mehsud. Largely a loose federation of various tribal and regional faction united by Mehsud, the group is located mainly in strongholds along the northwestern tribal belt, where the militants are also believed to be providing safe havens for senior al-Qaida leaders, including bin laden.

Mehsud was killed in an Aug. 5 CIA missile strike in northwestern Pakistan. He was replaced by his military chief, Hakimullah Mehsud. U.S. officials said it is hard to determine the precise number of Pakistani Taliban along the border, but they say it's in the thousands.

_Al-Qaida: Established by bin Laden in 1988 with the stated goal of uniting Muslims to defeat the West and form an Islamic caliphate. The group is behind a number of high-profile attacks including the October 2000 bombing of the USS Cole and the Sept. 11 attacks that saw 19 al-Qaida members hijack four airlines and slam them into the World Trade Center towers, the Pentagon and the Pennsylvania countryside.

While weakened and diminished in Iraq, the group has spawned franchises or affiliates in East Africa, Somalia, Indonesia and other locations around the world. Officials say there are a couple hundred in the Afghanistan-Pakistan region, including bin Laden and his chief deputy, Ayman al-Zawahiri.