South Sudan's civil war began on the night of December 15, 2013, when a firefight erupted between soldiers serving in the presidential garrison in the capital city, Juba. "Between" is an important word. The battle pitted soldiers from the Dinka tribe (largest in South Sudan) against soldiers in the Nuer tribe (second largest). The government, led by president Salva Kiir, a Dinka, and the rebels, led by Riek Machar, a Nuer, agreed to their first ceasefire on December 31. Fire and combat, however, never ceased. Instead, it spread. Every ceasefire and peace declaration since has failed to hold. Poor communication...