After the Tullahoma campaign, the two armies adopted their previous policy of remaining stationary. Each began to gather forces and equipment for a future struggle. The Union Army occupied a line from Winchester to McMinnvillethe same territory the Confederates had occupied previouslywhile the Confederate General Bragg established his headquarters at Chattanooga. There the Army of Tennessee strengthened its defensive position and prepared to close the "gate" to further advances of the Army of the Cumberland.
During July and August, Halleck again urged Rosecrans to move against Bragg's forces, but Rosecrans failed to budge. In the latter's judgment, three things were needed to insure a successful campaign. The first was ripe corn which would not be ready until August; the second was the repair of the railroad to the Tennessee River; and the third was support for his flanks. In spite of the constant flow of dispatches from Halleck to Rosecrans, it was not until August 16 that he began his movement southward to cross the river.
As Rosecrans moved toward the Tennessee River and Chattanooga, another Union army under command of General Burnside entered east Tennessee to threaten Knoxville. General Bragg, supposing that the two armies would join forces to attack him, made urgent appeals for help. Though the shortage of manpower at this time was a major problem of the Confederacy, troops were sent hurrying to Bragg from several directions.
Rosecrans' strategy, after viewing several possibilities, was to cross the river below Chattanooga, turn the Confederate left and interrupt his opponent's communications and supply line from Atlanta. This movement if successful would effectively cut all railroad lines to Chattanooga, and Bragg would find himself shut in between Burnside on the north and east and Rosecrans on the west and south. To deceive Bragg as to the point of crossing the Tennessee River, Rosecrans sent Hazen's and Wagner's infantry brigades, Wilder's mounted infantry, and Minty's cavalry, all under the command of Brig. Gen. William B. Hazen, to make a feint at the river north of the town and to annoy the enemy as much as possible.
The ruse was successful, and so thoroughly was Bragg deceived into thinking the attack would come from upstream on the north side of the Tennessee, he left the crossings below Chattanooga practically unguarded. Rosecrans with the bulk of his army then crossed the river in the vicinity of Bridgeport and Caperton's Ferry, Ala., and Shellmound, Tenn. By September 4, the Army of the Cumberland, thus meeting little opposition, was safely across a great barrier and was threatening Bragg from new positions.
When Bragg learned that the Union Army had crossed the Tennessee below Chattanooga and was threatening his supply lines, he decided after much deliberation to abandon his position and retreat southward.
Once the Union Army had crossed the river, Thomas' corps marched toward Trenton, Ga.; McCook's took the road to Alpine, Ga.; and Crittenden moved toward Chattanooga. On the 9th of September, Rosecrans, believing the Confederates to be in full retreat, ordered McCook to press forward toward Alpine, covered by the cavalry, and make attempts to cut Bragg off; Crittenden to garrison Chattanooga with one brigade and pursue Bragg by way of the Ringgold Road with the rest of his force; and Thomas to continue toward Trenton.
In order to understand the importance of the movements of both commanding generals, the geography of the country must be considered. When the Union commanders climbed to the top of the Lookout Mountain range and viewed the country, they began to have misgivings about their divided army. Thomas and McCook, 20 and 40 miles southwest of Chattanooga, respectively, found themselves on a mountain ribbed by ridges and hills, more than 1,000 feet above the valley floor. The few roads which ran over the mountain were narrow, rough, stony, and unusually steep.
Thomas, looking to the east, saw Pigeon Mountain, a spur that juts off Lookout Mountain and veers in a northeastwardly direction. The acute angle of these diverging mountains forms McLemore's Cove. Running into this cove from the northeast and ending there is the southern extremity of Missionary Ridge which begins immediately east of Chattanooga. Here, also, originates Chickamauga Creek which gave the ensuing battle its name.
As the two Union corps moved eastward they found the country sparsely populated. There were a few farms, but most of the land was covered with cedar thickets and tangled undergrowth. The roads connecting farm and village were dry and dusty.
The Union Army was now split into three distinct columns with its flanks more than 40 miles apart. In mountainous terrain, this made it impossible for them to support one another. In the period September 1012, corps commanders began to receive reports that a large Confederate force was at LaFayette, Ga. It was Bragg's army. He had not retreated as far south as Rosecrans had thoughthe had stopped at LaFayette behind Pigeon Mountain. There he concentrated his army and awaited reinforcements.