Darfur cannot be solved properly without taking into consideration the regional context.
Hundreds of thousands of displaced Darfuris languish in refugee camps across the east of Chad with no realistic prospect of going home. They will not want to return while there is still a risk of conflict between Darfur rebel groups and the Sudanese government, and this in turn cannot be guaranteed without realising the role the Chadian rebels play in regional destabilisation.
Twice now, in 2006 and 2008, Chadian rebels came within a kilometre of the Presidential Palace in N'Djamena and President Deby only survived by the skin of his teeth. These rebels were armed by Sudan, in a bid to take out Deby, who in President Bashir's eyes was not doing enough to stop the JEM and other Darfur rebels from using Chadian territory for rear bases.
Beyond their apparent role as 'Sudanese mercaneries' (Deby's stock phrase), the Chadian rebels do seem to articulate legitimate grievances - they are unhappy about the lack of democratic space in Chad, and Deby's almost total control of oil revenues, which ought to be being used for social development.
Without serious US engagement with Chad at a political level - in a similar vein to its commitment to Sudan - the problems in the east and Darfur, which are inextricably linked, will retain the potential to 'spillover' at any time.