The 123rd was initially assigned to the Army of the Ohio from September 1862 to November 1862, and then to the Army of the Cumberland from November 1862 to June 1865. The commanding general of the Army of the Cumberland was William Rosecrans. The regiment was in the 1st Brigade with Colonel Albert S. Hall, which was part of the 5th Division commanded by Brigadier General Joseph J. Reynolds, reporting to Major General George Henry Thomas. In February and March 1863, it was converted to mounted infantry. The 123rd Illinois' brigade became known as "Wilder's Lightning Brigade" commanded by Wilder. The brigade remained the 1st Brigade of the 5th Division, XIV Army Corps after its conversion. During this conversion, Spencer repeating rifles, invented by Christopher Spencer, were adopted as the command's primary weapon. 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. The weapon was estimated to allow the regiment to deliver five t seven times the firepower of muzzle-loading opponents. The 123rd first used its new rifles in the Battle of Hoover's Gap. The brigade showed the advantage of their speed despite the weather by reaching the gap nearly 9 miles ahead of Thomas's main body. 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. The brigade surprised Confederate Colonel J. Russell Butler's 1st (3rd) Kentucky Cavalry Regiment at the entrance of the gap. After driving them through the gap, the brigade found it outnumbered four-to-one. The brigade entrenched and held this position. The brigade, supported by Brig. Gen. Bushrod Johnson's brigade and some artillery, assaulted Wilder's position, but was driven back by the concentrated fire of the Spencers, losing 146 killed and wounded (almost a quarter of his force) to Wilder's 61. Colonel James Connolly, commander of the 123rd Illinois, wrote:  During the calamitous Chickamauga Campaign|Tullahoma Campaign, the 123rd and the rest of the Lightning Brigade, were one of the few positive results. The 123rd was sent to defend Alexander's Bridge over the Chickamauga on 17 September. The next day, 18 September, the Lightning Brigade blocked the crossing against the approach of W.H.T. Walker's Corps. Feeling quite confident in the advantage their Spencer repeating rifles gave them, the brigade held off a brigade of Brig. Gen. St. John Liddell's division, which suffered 105 casualties against Wilder's superior firepower.