The thing required was stupendous, but the results show it was not impossible. Sixty miles from the Union army was the Tennessee River and Cumberland Mountains. Both run from north-east to south-west. There are in these lofty mountain ranges occasional gaps, through which the great east and west traffic of the country takes place. Chattanooga, in 1863 a town of fifteen hundred inhabitants, is in the most important of these gaps—the one through which passes the Tennessee River and an important net-work of railroads. The town is right in the mountains, twenty-five hundred feet above the sea-level, and was strongly fortified, and practically impregnable to assault. Along the north-west front of the town runs the river, which would have to be crossed by the Union forces. On the southern side of the river, below Chattanooga, are three parallel ranges: Sand Mountain, Lookout Mountain, and Pigeon Ridge,—the valleys between the ridges running up to the gap at Chattanooga. North-east of the town the ridges begin again, and the general configuration of the country is similar. Chattanooga was south-east from where the Union army was situated. The town was the lock, and Bragg’s army the key, to the door to Georgia, Virginia, and the Carolinas. To unlock this door was the task before the Army of the Cumberland. But the problem of Rosecrans’s advance contained other complications beside the deep river, the lofty mountains, and the heavy fortifications. His army had to depend for its supplies upon Louisville, Kentucky, and the slender line of railway from that place. Every advance necessitated the weakening of his army by leaving strong detachments to preserve this communication; while, on the other hand, Bragg, already reinforced, would grow stronger all the time as he fell back on his reserves. It is reasonable to suppose that the reason Garfield had urged the advance toward Chattanooga was that he saw a way in which it could be made. When the peremptory order came, a plan for the advance was projected, which, though vaster and more complicated than that of the Tullahoma campaign, contains the same elements, and shows itself to have been the work of the same mind. It was, indeed, a continuation of the same campaign. The plan was Rosecrans’s, because he adopted it. It was Garfield’s, because he originated it. The theory of the advance was to pass the enemy’s flank, march to his rear, threaten his line of supplies and compel him, by military strategy, to evacuate Chattanooga, as he had Shelbyville and Tullahoma. The door would thus be unlocked, and Bragg’s army driven from its last fortification to the open country. The details of the plan, as prepared by Garfield, will appear as the advance is explained. On August 16th began the movement of the army across the mountains toward the Tennessee River. The paramount effort in the manner of the advance was to deceive the enemy as to the real intention.