Meanwhile, a complete reorganization of the Federal forces in the West was effected. Under the title of the Military Division of the Mississippi, the Departments of the Ohio, the Cumberland, and the Tennessee were united with Grant as general commanding, and Rosecrans was replaced by Thomas at the head of the Army of the Cumberland. A hurried concentration of the Federal forces was now ordered by General Halleck. Hooker with fifteen thousand men of the Army of the Potomac came rapidly by rail to Bridgeport. Sherman, with a portion of his army, about twenty thousand strong, was summoned from Vicksburg and at once embarked in steamers for Memphis. General Grant decided to assume personal charge of the Federal forces; but before he reached his new command, Thomas, ably assisted by his chief engineer, General W. F. Smith, had begun to act on a plan which Rosecrans had conceived, and which proved in the end to be a brilliant conception. This was to seize a low range of hills known as Raccoon Mountain on the peninsula made by a bend of the river, on its south side and west of Chattanooga, and establish a wagon road to Kelly's Ferry, a point farther down the river to which supplies could be brought by boat from Bridgeport, and at the same time communication effected with Hooker. A direct line was not only secured to Bridgeport, but Hooker advanced with a portion of his troops into Lookout Valley and after a short but decisive skirmish drove the Confederates across Lookout Creek, leaving his forces in possession of the hills he had gained. The route was now opened between Bridgeport and Brown's Ferry; abundant supplies were at once available and the Army of the Cumberland relieved of its perilous position. Unlike the condition which had prevailed at Chickamauga, reënforcements from all sides were hastening to the aid of Thomas' army; Hooker was already on the ground; Sherman was advancing rapidly from Memphis, and he arrived in person on November 15th, while Burnside's forces at Knoxville offered protection to the left flank of the Federal army. The disposition of the Confederate troops at this time was a formidable one; the left flank rested on the northern end of Lookout Mountain and the line extended a distance of twelve miles across Chattanooga Valley to Missionary Ridge. This position was further strengthened by entrenchments throughout the lowlands. Despite the danger which threatened his army from the converging Union forces, General Bragg determined to attack Burnside and despatched Longstreet with twenty thousand of his best troops to Knoxville. His army materially weakened, the Confederate general continued to hold the same extended position, although his combined force was smaller than had opposed Rosecrans alone at Chickamauga.