From Alfred McCoy, on the fact that opium farming generates 50% of Afghanistan's GDP and supports 20% of its population:
To understand the Afghan War, one basic point must be grasped: in poor nations with weak state services, agriculture is the foundation for all politics, binding villagers to the government or warlords or rebels. The ultimate aim of counterinsurgency strategy is always to establish the state's authority. When the economy is illicit and by definition beyond government control, this task becomes monumental. If the insurgents capture that illicit economy, as the Taliban have done, then the task becomes little short of insurmountable.
That's from "The Opium Wars in Afghanistan," a brief history of the three Afghan wars of the past three decades.