Everything about Staff Sgt. Adin Salkanovic says soldier. His clear, deliberate speech, close-cropped hair and serious demeanor give him an air of authority usually reserved for those with a lifetime of military experience, not five years of U.S. Army service. Those five years don’t tell the whole story, though. When he was only 14, Salkanovic spent a year in the Bosnian army, fighting Serbian forces that surrounded his hometown of Sarajevo for 3½ years between 1992 and 1995. “I volunteered many times but because of my age they said, ‘no,’” said Salkanovic, 24, who now serves as a scout with...