Later in the summer, taking advantage of the lull in operations elsewhere, the Confederate leaders sent Longstreet's splendid corps of veterans from Virginia, and that part of Johnston's army which had been paroled, together with such detachments as could be got from Alabama, to reinforce Bragg, who had been driven by Rosecrans from Middle Tennessee to Northern Georgia. Turning fiercely upon his over-confident pursuer, as soon as his reinforcements were at hand, Bragg struck a staggering blow at Chickamauga, which not only came near giving Chattanooga back to him, but filled the northern states with consternation. The war was not only not ended, but had burst forth with renewed vigor. Reinforcements in large numbers were hurried forward from all parts of the country to Chattanooga. Hooker, with Howard's and Slocum's corps, was sent out by rail from Virginia, while the greater part of Grant's Army of the Tennessee was withdrawn from the lower Mississippi, where it was resting after the capture of Vicksburg, and marched over-land from Memphis to the same place. The separate departments in the Mississippi Valley were consolidated into a military grand division, under the supreme command of General Grant, and what turned out to be of almost equal importance was the fact that Brigadier General William F. Smith was relieved from service in West Virginia, where he had been recently assigned to duty, and sent to contribute his part towards strengthening the national grasp upon the vast region of which Chattanooga was justly considered the strategic center. Whatever the government at that time may have thought of him as a commander of troops, it is certain that it was willing to recognize and use his experience and marked intellectual resources as an engineer officer to their fullest extent. As it turned out, it could not have paid him a greater compliment, nor given him a better opportunity for distinction. His fame had gone before him, and on his arrival at Chattanooga, although he preferred the command of troops, he was assigned at once to duty as Chief Engineer of the Department and Army of the Cumberland. Fortunately this gave him the control, not only of the engineer troops and materials, and the engineer operations of that army, but carried with it the right and duty of knowing the army's condition and requirements as well as all the plans which might be considered for extricating it from the extraordinary perils and difficulties which surrounded it.