JALALABAD, Afghanistan -- U.S. Army Capt. Brian Waters was rounding a bend in the Pech River Valley last month when insurgents opened fire on his convoy. The Americans returned fire with machine guns and grenade launchers.

In the middle of the 75-minute firefight, with insurgent rounds slapping the dirt road, a cluster of Afghan children jumped onto the hood of Capt. Waters's Humvee and snatched the empty brass shells pouring out of the machine gun.

"They're really just fearless," Capt. Waters, a 30-year-old from Bonney Lake, Wash., said of the young scrap-metal collectors.

War upends society. It also creates an economic logic of its own, from the tens of millions of aid dollars going to Afghan road-building companies to the pennies that children earn by collecting empty shell casings cast off by foreign soldiers and selling them to scrap dealers.