John G. Nicolay, A Short Life of Abraham Lincoln, condensed from Nicolay and Hayes' Abraham Lincoln: A History

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: