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The Taliban today in Afghanistan is a markedly different movement from that of those warriors whose one-eyed leader, Mullah Omar, riding on a motorcycle, escaped capture from American forces in Kandahar in December 2001. Mullah Omar is still their leader, even though, as a senior Afghan intelligence official told TIME, he is thought to be hiding across the border in Pakistan, moving between the towns of Quetta and Zob in the scorched Baluchistan desert. Nowadays, though, the Taliban encompasses a vast and disparate array of players. A look at who they are is key to understanding why they are gaining ground against 63,000 U.S. troops and their NATO partners after eight years of guerrilla war.

The Taliban is not monolithic. It is composed of several layers: a...