CAMP PENDLETON -- As a sniper, Sgt. Chad Cassady, a quiet Texan with a master's degree in political science, helped protect his fellow Marines during the battle in Fallouja, Iraq, in late 2004. But when insurgent mortars fell behind and in front of the Marines -- a tactic called "bracketing" -- Cassady and other Marines were forced to run across a rubble-strewn street. When another mortar landed in their midst, two Marines were knocked to the ground and Cassady was thrown into the courtyard of a home. The explosion collapsed his right lung, lacerated his liver and kidneys, and peppered...