RAMADI, Iraq - The young Marine had just shot a suspected insurgent and was walking back across the villa's rooftop when he keeled over from a terrific thud to the back of his head. A sniper had fired a single, well-aimed bullet that tore through the top of Lance Cpl. Richard Caseltine's helmet, traced a path along the edge of his skull and buried burning bullet fragments in the back of his neck. Less than a minute later, the 20-year-old from Aurora, Ind., was up on his feet — crouching, shaking and miraculously, still alive. "You expect when somebody gets...