According to this and associated articles, Chad was attacked by rebels they claim are backed by Sudan. Chad was attacked, not Sudan.
"NDJAMENA, Chad, Dec. 24 (UPI) -- Chad has accused neighboring Sudan of supporting a rebel attack on the town of Adre and says that "a state of war" exists.
About 100 people were killed when rebels in Chad attacked Adre last Sunday. In Friday's statement, the government of Chad also says that the Sudanese have been crossing the border to burn villages and steal cattle, killing civilians in the process, the BBC reports."
Chad urges UN to stem spread of Darfur conflict
Fri Dec 23, 2005 8:42 AM GMT
UNITED NATIONS (Reuters) - Chad accused neighboring Sudan on Thursday of trying to draw it into the civil war in Sudan's Darfur region and urged the U.N. Security Council to help prevent the conflict from spreading.