Sgt. Donald Hamblen was seriously injured on a routine exercise in 1962. And though his leg was amputated six inches below the knee, he fought to stay in the Corps and passed every kind of endurance test. Training other Marines for service in Vietnam in the supersecret Studies and Observation Group (SOG) in 1965, he trained teams of Vietnamese for clandestine missions in both North and South Vietnam. Wounded twice, he served 30 unbroken months of...