The jagged, 1,357-mile-long border between Sudan and the newly independent nation of South Sudan is the most contentious on the continent of Africa (see the map below). Tens of thousands of troops are massed on either side and, despite peace negotiations, border skirmishes recently broke out, in which scores of soldiers were killed. The two countries are bitterly divided over how to split oil profits and how to resolve contested territory.

The war between the northern and southern Sudanese is one of the longest and most complex in Africa, driven by religious schisms, racial politics, oil, and an especially convoluted colonial legacy. The first period of fighting began in 1955 and lasted until 1972. During the second period of conflict, between 1983 and 2005, more than two million people were killed and hundreds of thousands of starving people became refugees, including the Lost Boys, southern Sudanese children who were cast off to wander for hundreds of miles across the savannah, dodging lions and bombers. One of the terms of the peace agreement that ended the fighting in 2005 was a referendum held in 2011 in which southern Sudanese voted overwhelmingly for independence from the north.

Within this broader conflict, the rebellion in the Nuba Mountains may be the most intractable. It is also the one causing the most intense violence and suffering right now. The Nuba rebel army, based within Sudan but with support from South Sudan, has been willing to fight far better equipped Sudanese government forces.

The Nuba people were celebrated by Leni Riefenstahl in a book of photographs, The Last of the Nuba (1973). Riefenstahl, the famous German filmmaker and confidante of Hitler, was drawn to the Nuba because of the vivid quality of their traditions, including public wrestling, which lasted well into the 1970s and continue to some degree to this day. Their territory, the Nuba Mountains—really hills about three thousand feet high—lies just across the border from South Sudan. For centuries the Nuba retreated into these hills to escape slave raiders. The Nuba are not Arab, like the rulers in Khartoum, and many are not Muslim; they have long seen themselves as closer to the ethnic groups, many with animist beliefs, of what is now South Sudan. They are officially Sudanese citizens, but they are seen by the Sudanese government in Khartoum as subversive enemies and they have been mercilessly bombed and starved.

Last summer, I boarded a plane carrying medical supplies into the Nuba Mountains. The pilot was a chain-smoking Bulgarian woman who lives in Nairobi with exotic animals, including monkeys and chimps, which she routinely rescues from the war zones where she flies. She had filed a misleading flight plan that morning because no one was supposed to know our destination,…