BAGHDAD, Iraq — On a cool Wednesday night in Baghdad, with the stars bright overhead, Pfc. Ernie Ritacca sits in his mobile radar van, the door open, waiting for signs of “The Mad Mortarman.” Ritacca, 19, of Bullhead, Ariz., is with the 1st Armored Division’s Battery D, 1st Battalion, 94th Field Artillery Regiment. His van and a few other vehicles are tucked behind an earthen embankment near the flightline of Baghdad International Airport. “The Mad Mortarman” is the nickname some in his unit use for the faceless Iraqi insurgents who have been keeping a steady routine of hit-and-run attacks on...