When then-Lance Cpl. Javier Ortiz came home from a secret mission in Syria, the ghost of a dead girl appeared to him in his kitchen. She was pale and covered in chalky dust, as if hit by an explosion, and her eyes stared at him with a glare as dark and heavy as oil.The 21-year-old Marine was part of an artillery gun crew that fought against the Islamic State group, and he knew that his unit’s huge cannons had killed hundreds of enemy fighters. The ghost, he was sure, was their revenge.A shiver went through him. He backed into another...