During the month of November it became certain that General Longstreet had been detached from the army of General Bragg, with the design of attacking General Burnside, at Knoxville. After fortifying the city in such manner as to insure confidence in his ability to maintain it, General Burnside advanced to meet his antagonist, cautiously luring him on, to invite his withdrawal from the support of Bragg, and finally falling back within the defences of Knoxville, on the 17th of November, with the rebel army pressing close upon his rear. The Union army was informed that there was now to be no more retreating. The old defences were strengthened and new ones erected, rifle-pits dug, and trees felled to resist the approach of the besiegers. The city lies on the north bank of the Holston river, and a range of hills protects it on the west. The rebels, therefore, prepared to attack the defences from the north and east. Their line extended in a circular form, with their right touching the river, thus cutting off all water communication, and the supplies of the army were thenceforward to be obtained by forage trains alone. A long siege was not feared by General Burnside, as he had the promise of assistance from General Grant, who had just been called to the command of the army confronting Bragg. The brilliant victories of Lookout Mountain and Missionary Ridge now enabled General Grant to hasten forward the expected succor. Of this fact Longstreet soon became informed, but as he had confidently anticipated the capture of the city and its defenders, he was loth to leave his anticipated prey without a desperate and hasty effort for its seizure. He would try to carry Knoxville by storm. One of the principal defences of the place was called Fort Sanders, in honor of a brave general who had fallen in the early part of the siege. It was situated on a high hill on the northeast corner of the town, and was composed of well-planned earthworks. This fort commanded the approaches to the city both from the north and east, and its capture was a necessity before the assailants could enter Knoxville. The sides of the hill had been covered with a dense forest of pine, which had been felled, and now presented an abattis or network of brush or timber, almost impassable, to within two or three hundred yards of the fort, where a cleared space intervened, affording free range for grape or canister. The works consisted of a ditch and parapet.