Written with Nicky Woolf. Original available here.
The popular demand for southern Sudan to split from the rule of Khartoum and the north is enormous, and few now doubt the south will choose to be Africa's newest state. But tensions are intensifying along the proposed border, which runs through some of the most fertile land in the country.
The comprehensive peace agreement was signed in 2005 and brought an end to a 23-year civil war. It was agreed that if the south was to secede, its northern boundary would follow the upper border of the southern states as drawn in 1956, on the day of independence.
It is a line drawn with a thick pen on an old map, and takes little account of the reality on the ground, of tribal territories in permanent flux.
That these territories are not properly represented on any map is hardly surprising. The boundaries between the 20 or so border tribes are fluid. Like the tribes themselves, they follow the ebb and flow of the seasons.
For the majority of the tribes living along the border, cows are at the heart of their existence. To be a Nuer man is to own cattle. Until then, one is a boy. Among the Dinka, sacrifices of cows announce every marriage, and life is lived in a particularly intimate union with their herds.
The tribes spend much of the year settled in villages, grazing the cattle that are their lifeblood, but come the dry season, many tribes become nomadic, making long journeys in search of pasture.
Ground that is fertile one year may not be the next. Oil installations and agricultural projects have pushed northern Arab tribes further south in search of water, increasing the pressure on the southern tribes, who rely on the same rivers; rivers that run right through the border region. For everyone, dry-season grazing land has become harder to find.
Inter-tribal politics is extremely delicate. Historically, disputes are resolved by meetings of tribal elders. These meetings arrange grazing routes according to a subtle arithmetic that includes marriages, a changing ecology and prior cattle raids.
In Northern Bahr el Ghazal, the 1956 line places the Bahr el Arab river north of the border; a crucial water source for the Dinka Malwal, whose territory lies in southern Sudan.
The Comprehensive Peace Agreement mandates that grazing rights will be unaffected by the national border, but this is unlikely to be the case. Some of the Rizeigat, Arab nomads to the north of the river, are already claiming exclusive rights to an area in which the Malwal previously had grazing rights.
This pattern is being repeated across the borderlands. The national boundary has the potential to transform co-existence into the fixed lines of enemies.
Life along the border is hard, and often violent. Almost all the tribes are heavily armed. Skirmishes are common. Cattle raiding has always occurred between tribes like the Rizeigat and Malwal, but the second civil war dramatically changed relations between them.
The Rizeigat formed into Murahalin militias, at the behest of the northern government, and the destruction they wreaked in the south was unparalleled. In the south, many of the Malwal joined the Southern People's Liberation Army.
Today, with a national boundary about to be placed straight between them, international politics will effect no less of a transformation. If the south succeeds in secession, the tribes will have two nervous – and historically capricious – national governments to deal with.
If the institutions we expect to see at borders – passport controls and soldiers – are imposed post-independence, then cattle raiding across those borders could become an international incident, and may be interpreted by the government of either side as an act of war.
Several proposals have been suggested to prevent outbreaks of violence. One of the most prominent is a soft border, in which there would be a demilitarised zone which nomadic groups could cross at will.
However, history suggests this will be fraught with difficulty. Following 23 years of violence, relations between many tribes are at an all-time low.
Secession is overwhelmingly desired here in the south, but if the nascent Southern Sudanese state is to be viable, both countries must work to accommodate the border tribes, and ensure their ways of life are made sustainable.
There is hope. A meeting between the Rizeigat and the Malwal was held in January 2010 in Aweil, which affirmed that both should have "unhindered access to natural resources inside both communities' land".
It remains to be seen if such goodwill can survive a national border.