Funding comes from many sources: governments, donors, and Afghan poppy farmers, in exchange for protection for their fields.
"A large majority ... is thought to derive from wealthy individuals living in Arab Gulf states," said Mr. DuPee. Insurgents may also use the hajj – the Muslim pilgrimage to Mecca – as a time to raise funds, he adds. These ties to Gulf-based militants may account for Al Qaeda's influence over some groups.
Pakistan is also widely believed to provide covert funding to the Afghan insurgents.