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." And to Rosecrans he telegraphed directly, bidding him be of good cheer, and adding: "We shall do our utmost to assist you." To this end the administration took instant and energetic measures. On the night of September 23, the President, General Halleck, several members of the cabinet, and leading army and railroad officials met in an improvised council at the War Department, and issued emergency orders under which two army corps from the Army of the Potomac, numbering twenty thousand men in all, with their arms and equipments ready for the field, the whole under command of General Hooker, were transported from their camps on the Rapidan by railway to Nashville and the Tennessee River in the next eight days. Burnside, who had arrived at Knoxville early in September, was urged by repeated messages to join Rosecrans, and other reinforcements were already on the way from Memphis and Vicksburg.