Rosecrans' forward movement from Murfreesboro, in the early summer of 1863, forced Bragg over the Cumberland Mountains and across the Tennessee. The Confederate leader destroyed the railroad bridge at Bridgeport and entrenched himself in and around Chattanooga. The three Federal corps under Crittenden, Thomas and McCook crossed the Tennessee without meeting resistance, and began to endanger Bragg's lines of communication. But on September 8th, before their moves had been accomplished, Bragg abandoned his stronghold. Crittenden the next day marched around the north end of Lookout and entered the town, while Hazen and Wagner crossed over from the opposite bank of the Tennessee. Rosecrans believed that Bragg was in full retreat toward Rome, Georgia, and Crittenden, leaving one brigade in Chattanooga, was ordered to pursue. Bragg encouraged his adversary in the belief that he was avoiding an engagement and sent spies as deserters into the Federal ranks to narrate the details of his flight. Meanwhile, he was concentrating at Lafayette, about twenty-five miles south of Chattanooga. Hither General S. B. Buckner, entirely too weak to cope with Burnside's heavy column approaching from Kentucky, brought his troops from Knoxville. Breckinridge and two brigades arrived from Mississippi, while twelve thousand of Lee's veterans, under Lee's most trusted and illustrious lieutenant, Longstreet, were hastening from Virginia to add their numbers to Bragg's Army of Tennessee. The three corps of the Union army, as we have seen, were now separated over a wide extent of territory by intervening ridges, so intent was Rosecrans on intercepting the vanished Bragg. But the latter, by no means vanished, and with his face toward Chattanooga, considered the position of his antagonist and discovered his own army almost opposite the Federal center. Crittenden was advancing toward Ringgold, and the remoteness of Thomas' corps on his right precluded any immediate union of the Federal forces. Bragg was quick to grasp the opportunity made by Rosecrans' division of the army in the face of his opponent. He at once perceived the possibilities of a master-stroke; to crush Thomas' advanced divisions with an overwhelming force. The attempt failed, owing to a delay in the attack, which permitted the endangered Baird and Negley to fall back. Bragg then resolved to throw himself upon Crittenden, who had divided his corps. Polk was ordered to advance upon that portion of it at Lee and Gordon's Mills, but when Bragg came to the front September 13th, expecting to witness the annihilation of the Twenty-first Corps, he found to his bitter disappointment that the bishop-general had made no move and that Crittenden had reunited his divisions and was safe on the west bank of the Chickamauga. Thus his splendid chances of breaking up the Army of the Cumberland were ruined.