The same year that North and South Sudan signed an armistice halting their civil war, neighbouring Chad declared its own war on the North. Now, five years later, Khartoum and N'Djamena have a joint security agreement along their common border, most of which runs along the troubled western Darfur region. It is a tenuous line of stability that is sorely needed.
As South Sudan prepares to go to the polls in January to vote on independence, fears are rampant of renewed hostilities, which abated after the 2005 ceasefire. In reality, strife was never too far distant from the region. Darfur has been a weeping sore of instability, not to mention human rights violations, that has troubled the security of neighbouring Chad, Libya and the Central African Republic.
Last week, Minni Minnawi, the leader of the Sudan Liberation Army, the only Darfur rebel group to have signed a peace agreement with Khartoum, denounced the deal and his supporters resumed fighting with government forces at the weekend. Mr Minnawi has fled to South Sudan and appears to be trying to mobilise at least tacit support from leaders in Juba, as well as a new rebel bloc in Darfur.
Instability has a way of troubling one's neighbours. It was cross-border raids from Darfur that drew Chad into its short war in 2005. The country struggles to deal with 270,000 Darfur refugees, 150,000 Chadian internally displaced people in the east, and 65,000 Central African refugees in the south.
Both Sudan and Chad have a history of becoming host to rebel groups hostile to the other's government - including the notorious janjaweed militia. That makes the joint patrols along the open border all the more important.
Members of the African Union face a crucial test of their diplomacy as the uncertainty of post-referendum Sudan approaches. "The continent has never witnessed the demarcation of borders as a result of an impending referendum that could result in secession," Abdullah el Sadiq, the chair of Sudan's border demarcation dispute, told the London-based daily Asharq Al-Awsat.
Sections of the border between North and South Sudan, not to mention the contested Abyei region, are still uncertain; in truth, all of the borders in the area are porous. Urgent diplomacy, within Sudan and among the nine neighbouring countries, is needed to prevent instability from spilling over into bloodshed.