NASIR, South Sudan—When she was a girl, in the 1960s, Sarah Kier's parents fled southern Sudan with her. The Sudanese civil war, in which black-African inhabitants from the country's south were fighting for autonomy against an oppressive Arab-dominated government in the north, had been going on, without respite, for more than a decade. Kier's family moved to the hills of western Ethiopia. When she grew older, Kier became aware of the south's struggle. She joined the southern rebel militia, the Sudanese People's Liberation Army (SPLA), and became a battlefield medic. She spent much of her early life around violence and death.

On a hazy afternoon this past April, Kier was in the passenger seat of an old Land Cruiser, moving through