Bragg occupied Chattanooga, the strategic importance of which has been explained in an earlier chapter. The position was practically impregnable by any form of direct assault; for Rosecrans to have hurled his army against it would have been an act of well-nigh suicidal folly. In such an assault he must have lost ten men for every one of his enemy's men whom he could hope to put hors de combat. Yet it was necessary for Rosecrans to get Bragg out of Chattanooga. In order to do so he pushed a part of his army southward, threatening an invasion of Georgia. That state was defenseless except in so far as Bragg's army defended it. Rosecrans's movement, therefore, quickly compelled Bragg to withdraw from his strong and threatening position in order to head off what he supposed to be that southward movement that Sherman afterward made, and that is known in history as "the march to the sea." As soon as he quitted Chattanooga, Rosecrans occupied that place about the middle of September. It was apparently good policy for Rosecrans instantly and aggressively to follow his foe, and he did so in spite of the fact that his three corps were dangerously separated at a time of heavy rains, when the roads were bad, streams out of their banks, marching difficult, and promptitude of movement impossible. In the meantime Bragg had been heavily reinforced by Longstreet's corps sent out by Lee to save this situation. Thus strengthened Bragg turned about with intent to assail his adversary, and perhaps to destroy him. On the nineteenth of September the two armies met on the banks of Chickamauga creek. There for two days raged one of the great battles of the war. Rosecrans brought into the action about 55,000 men and Bragg had perhaps 10,000 more. It was Bragg who made the attack. As the lines lay, the Confederate right and the Federal left extended toward Chattanooga.