(NYT Magazine) – Chuol was only 5 in 2011, the year South Sudan, after decades of war, became the world’s newest nation. He was living with his parents, grandparents and other relatives in a village near the city of Leer, not yet old enough to understand the hope and joy sweeping through the small East African country. But that optimism turned to despair two years later, when a power struggle between President Salva Kiir and the former vice president Riek Machar triggered a civil war. The battle for control of the country — and its oil fields — pitted the nation’s two largest ethnic groups against each other: The Dinka aligned with the president, and the Nuer aligned with the vice president.