Additional roads were essential for safety but to gain them the Confederates had to be attacked and a heavy force was therefore ordered to seize and hold a point known as Brown's Ferry. This relieved the situation at once and meanwhile the new commander had hurried a special messenger to Sherman, ordering him to drop everything else and march his Vicksburg veterans toward Chattanooga without an instant's delay. The advance of this strong reënforcement was promptly reported to Bragg, who saw at a glance that unless it could be stopped there was every prospect that his Chattanooga victims would escape. He accordingly determined upon a very bold but very dangerous move. Not far away lay General Burnside and a small Union army, guarding the important city of Knoxville, Tennessee, and against this the Confederate commander dispatched a heavy force, in the hope that Grant would be compelled to send Sherman to the rescue. But the effect of this news upon Grant was very different from Bragg's expectations, for realizing that his adversary must have seriously weakened himself in sending the expedition against Burnside, he ordered Hooker, whose 16,000 men were already on hand, to make an immediate attack with a force drawn from various parts of the army, and on November 24, 1863, after a fierce engagement known as the battle of Lookout Mountain, the Union troops drove their opponents from one of the two important heights commanding Chattanooga. In this success Sherman had effectively cooperated by attacking and holding the northern end of Missionary Ridge and Grant determined to follow up his advantage by moving the very next morning against this second and more formidable range of hills. Therefore, ordering Hooker to attack the Confederate right on Missionary Ridge and get in their rear at that point while Sherman assaulted their left, he held Thomas's troops lying in their trenches at the front awaiting a favorable opportunity to send them crashing through the center. The main field of battle was plainly visible to the silent commander as he looked down upon it from a hill known as Orchard Knob, and he watched the effect of the attacks on both wings of the Confederate line with intense interest. Reënforcements were evidently being hurried to the Confederate right and left and Hooker, delayed by the destruction of a bridge, did not appear at the critical moment. Nevertheless, for some time Sherman continued to advance, but as Grant saw him making slower progress and noted the heavy massing of troops in his path, he ordered Thomas's waiting columns to attack the center and carry the breastworks at the foot of Missionary Ridge.