This is what bad intelligence produces: a girl's dress, its embroidery stained dark red with blood, lying amid the rubble of a bombed-out building. Men wandering through the debris, gesturing to show where people were dancing when the bombs began to fall. And a U.S. special-forces soldier, who is said to have surveyed the scene and asked, "Why did we do this?"
It was a wedding party on a late December night. But from the air, it looked to the pilots like what their intelligence source had claimed: a gathering of al-Qaeda terrorists. Dozens of cars had converged on Qila-Niazi, a...