OBO, Central African Republic—Obira Julius was stretched out on the ground, taking a breather on a slog from one jungle hideout to another, when he heard the voice in the sky. It was disembodied but familiar, a voice from his lost childhood, suddenly floating down to him from a loudspeaker on a passing American helicopter.

At first Mr. Obira wasn’t sure. After all, he had last heard that voice 13 years earlier, when he was just 5, on the day the rebels snatched him and two brothers on the way to their grandfather’s house....