On October 1, having been assigned to the Thirty-third Brigade (General William R. Terrill), Fourth Division (General James S. Jackson), in McCook's Corps, the regiment started on the march under Buell, southward through Kentucky, after Bragg, who had turned back, and up to this time the regiment never had battalion drill, and hardly an attempt at company drill, as all the officers, except the colonel, were "raw recruits".

Just 19 days after leaving Mattoon, the regiment engaged in the Battle of Perryville, where 36 men were killed in action and 180 wounded.

Following the devastating bloodshed the 123rd was assigned to protect the railroad bridge across the Green River at Munfordville, Kentucky, in November and December 1862.

The new increase in firepower that the Spencer gave, allowed the 123rd and its brigade mates to see off numerically superior Confederate infantry and cavalry in several engagements.

[5] The brigade showed the advantage of their speed despite the weather by reaching the gap nearly 9 miles ahead of Thomas's main body.

[6] Despite orders from the divisional commander, General Joseph J. Reynolds to fall back to his infantry, which was still six miles away, Wilder decided to take and hold the position.

Colonel James Connolly, commander of the 123rd Illinois, wrote: As soon as the enemy opened on us with their artillery we dismounted and formed line of battle on a hill just at the south entrance to the "Gap," and our battery of light artillery was opened on them, a courier was dispatched to the rear to hurry up reinforcements, our horses were sent back some distance out of the way of bursting shells, our regiment was assigned to support the battery, the other three regiments were properly disposed, and not a moment too soon, for these preparations were scarcely completed when the enemy opened on us a terrific fire of shot and shell from five different points, and their masses of infantry, with flags flying, moved out of the woods on our right in splendid style; there were three or four times our number already in sight and still others came pouring out of the woods beyond.

Our regiment lay on the hill side in mud and water, the rain pouring down in torrents, while each shell screamed so close to us as to make it seem that the next would tear us to pieces.

[10][11][12] At around 14:00 on 19 September, the 123rd and its brigade spoiled the left column of the main rebel attack by severely mauling both Brig.

The attacking Confederates were surprised by the resolute, confident manner that the Lightning Brigade demonstrated in driving them back.

[13][14][15] During the Siege of Chattanooga, the Lightning Brigade disbanded, and four of its regiments shifted to the Cavalry Corps where they served for the remainder of the war.