Tennessee
At the conclusion of the Gettysburg Campaign, the Army of the Potomac pursued Robert E. Lee into Virginia, the XII Corps joining in the pursuit, and pushing forward until it reached the Rappahannock. While encamped there, on September 23, 1863, the XI and XII corps were detached from the army and ordered to Tennessee as a reinforcement for William Rosecrans, besieged in Chattanooga. The two corps were placed under the command of Maj. Gen. Joseph Hooker. Arriving in Tennessee, Geary's Division moved to the front, while Williams's Division was stationed along the railroad from Murfreesboro to Bridgeport. Geary pushed on in order to effect a junction with the beleaguered army at Chattanooga. On the night of October 27, his division bivouacked in Lookout Valley, in an advanced and isolated position, where he was attacked at midnight by a part of James Longstreet's command. But Geary had taken proper precautions against surprise, and Longstreet was repulsed, Geary receiving in this affair a prompt and gallant support from part of the XI Corps. General George H. Thomas, commanding the Army of the Cumberland, stated in his official report that "the repulse by Geary's Division of greatly superior numbers who attempted to surprise him, will rank among the most distinguished feats of arms of this war."
The midnight Battle of Wauhatchie was followed in the next month by the brilliant victory at Lookout Mountain, where the 2nd Division fought its famous "battle above the clouds". Geary was assisted in this engagement by Walter C. Whitaker's Brigade of the IV Corps. One of Whitaker's regiments, the 8th Kentucky, was the first to plant its flag on the summit of the mountain.
Read more about this topic: XII Corps (Union Army)