With this resolve, Bragg turned upon his antagonist but his effort at quick concentration was delayed by the natural difficulties of the ground.  By September 19, both armies were well gathered on opposite sides of Chickamauga Creek, eight miles southeast of Chattanooga; each commander being as yet, however, little informed of the other’s position and strength.  Bragg had over seventy-one thousand men; Rosecrans, fifty-seven thousand.  The conflict was finally begun, rather by accident than design, and on that day and the twentieth was fought the battle of Chickamauga, one of the severest encounters of the whole war.  Developing itself without clear knowledge on either side, it became a moving conflict, Bragg constantly extending his attack toward his right, and Rosecrans meeting the onset with prompt shifting toward his left.

In this changing contest Rosecrans’s army underwent an alarming crisis on the second day of the battle.  A mistake or miscarriage of orders opened a gap of two brigades in his line, which the enemy quickly found, and through which the Confederate battalions rushed with an energy that swept away the whole Union right in a disorderly retreat.  Rosecrans himself was caught in the panic, and, believing the day irretrievably lost, hastened back to Chattanooga to report the disaster and collect what he might of his flying army.  The hopeless prospect, however, soon changed.  General Thomas, second in command, and originally in charge of the center, had been sent by Rosecrans to the extreme left, and had, while the right was giving way, successfully repulsed the enemy in his front.  He had been so fortunate as to secure a strong position on the head of a ridge, around which he gathered such remnants of the beaten detachments as he could collect, amounting to about half the Union army, and here, from two o’clock in the afternoon until dark, he held his semicircular line against repeated assaults of the enemy, with a heroic valor that earned him the sobriquet of “The Rock of Chickamauga.”  At night, Thomas retired, under orders, to Rossville, half way to Chattanooga.

The President was of course greatly disappointed when Rosecrans telegraphed that he had met a serious disaster, but this disappointment was mitigated by the quickly following news of the magnificent defense and the successful stand made by General Thomas at the close of the battle.  Mr. Lincoln immediately wrote in a note to Halleck: 

“I think it very important for General Rosecrans to hold his position at or about Chattanooga, because, if held, from that place to Cleveland, both inclusive, it keeps all Tennessee clear of the enemy, and also breaks one of his most important railroad lines....  If he can only maintain this position, without more, this rebellion can only eke out a short and feeble existence, as an animal sometimes may with a thorn in its vitals.”