Zach Barrow, a 27-year-old Army gunner who rode in the same truck as Pfc. Bergdahl, described his shift.

“It seemed like he was this die-hard, Rambo-esque soldier who wanted to kick a— and take names who then became this Peace Corps kind of guy who wanted to help the people,” Mr. Barrow told The Wall Street Journal in his first interview about Sgt. Bergdahl.

Soldiers who trained with then-Pfc. Bergdahl described the arriving Army private from Idaho as a quiet loner who favored books on Buddhism over videogames. He told friends he was named after Chick Bowdrie, the tough Texas Ranger in author Louis L’Amour’s cowboy short stories.

Especially at first, Pfc. Bergdahl was eager to fight. In May 2009, shortly after he arrived in Afghanistan, he took part in a mission to rescue an Army unit stuck in the mountains after a roadside bomb had disabled one of its armored vehicles.

On the narrow mountain road, a vehicle in his convoy hit a roadside bomb, leaving his unit stuck in the middle of Taliban-dominated terrain for days. As the stranded soldiers grew anxious, waiting for commanders to come up with a plan, Pfc. Bergdahl fantasized about life in Afghanistan.