PICAYUNE, Miss. — With the rifle skills she honed in the Mississippi backwoods, Victoria Bauer had a path to escape the trap of drugs and dead-end jobs she saw most everywhere around her. Her future was in the Marines, she decided, and she had an idea about how to get there.

Across the way from her freshman algebra class, Ms. Bauer approached Steve Hardin, the retired Navy intelligence officer who guided the high school’s Junior Reserve Officers’ Training Corps, a leadership program sponsored by the U.S. military at high schools across the country. He welcomed her into the fold, she said, and seemed interested in how her family, which traced roots back to the Four Winds Cherokee of Louisiana, had been displaced by Hurricane Katrina.

Soon, her 45-year-old J.R.O.T.C. instructor was messaging her on Snapchat late into the night, telling her that it would “drive the guys crazy” if she wore a “small bikini” during the trip to their next out-of-state shooting competition. Then one night in 2015 as he drove her home from rifle practice, she told investigators, Mr. Hardin pushed his hand into her pants and penetrated her with his fingers — the start of what she said was months of sexual assaults. Ms. Bauer, who was 15 at the time, feared that resisting him would jeopardize her shot at advancement through the J.R.O.T.C. ranks or a military career.