Raiding among cattle-herding tribes is a traditional part of life in South Sudan, but in the past five years, the skirmishes have become more violent and unrestrained. Small armed bands that traditionally guarded their communities’ livestock have been drawn into bitter proxy battles between the country’s two largest tribes: the Dinka, who hold power in Juba, the national capital, and the Nuer. No longer limited to raiding each other’s villages and herds, these bands of well-armed tribal fighters have carried out massacres and atrocities, with women and children increasingly among the victims. The violence has worsened in both scale and frequency since 2013, according to South Sudanese who have witnessed the ravages of a civil war that started just two years after their country gained independence from Sudan.

The continued rise of local factions throughout global conflict zones, like these young Dinka and Nuer fighters in South Sudan, is a growing problem for humanitarian workers. “We’ve seen more nonstate armed groups emerge in the last seven years than in the previous 70 years,” says Brian McQuinn, an adviser on nonstate armed groups for the International Committee of the Red Cross and co-author of a new I.C.R.C. study on the topic. “All of these new groups that are emerging are structured in different ways.” For the most part, they are looser, with less top-down control. “The complexity of that is off the scale.” This causes a problem for humanitarian organizations whose standard method has been to go to the top commanders of a military or militia group, advise them on international humanitarian law and rely on the chain of command to enforce the rules throughout the ranks.