SAMARRA, Iraq — The sun was sinking at the desert’s edge when Sgt. Randall Davis spotted his target, an armed Iraqi on a rooftop about 300 meters away. “It was just getting dark. I saw a guy step in front of the light,” said the 25-year-old sniper. Davis knew he was watching another sniper by the way the man stepped back into the shadows and crept along the roofline to spy down on a squad from his unit — B Company, 5th Battalion, 20th Infantry Regiment. “Most people, when they get on a roof, will just move around and do...