This regiment was part of Illinois' response to President Lincoln's calls for 600,000 men in the summer of 1862, and was made up of the sturdy farmers, mechanics, clerks, and school teachers of Macon, Shelby, Christian, Wabash, Sangamon, Morgan, Schuyler, and Tazewell counties. Col. Jesse H. Moore, of Decatur, was most prominent in the work of recruiting the regiment, though ably assisted by Lt. Col. Wm. Kinman of Jacksonville, Maj. Geo. A. Poteet of Shelbyville, Capt. F.L. Hays of Decatur, and others. The several companies went into camp rendezvous at Camp Butler about the middle of August. The regiment was organized on August 26th, and was mustered into the service of the United States on the 13th of September, 1862. Having been armed and equipped for the service the regiment was ordered to the field on the 4th of October, and two days later reported to Maj. Gen. H.G. Wright at Cincinnati, and was placed in camp near Covington, Ky., under command of Brig. Gen. A.J. Smith. On October 18th it began its march to Lexington via. Falmouth, Cynthiana, and Paris, arriving at Lexington on the 28th. On the organization of the army of Kentucky the 115th was assigned to the 2nd brigade, composed of the 92nd, 96th and 115th Illinois and the 14th Kentucky, the brigade being commanded, while at Lexington, by Col. P.T. Swayne of the regular army, then colonel of the 99th Ohio, which was a part of the 3rd division, commanded by Brig. Gen. Absalom Baird, Maj. Gen. Gordon Granger commanding the army. On November 13th the regiment started to Richmond, Ky., where it remained until December 20th, Col. Moore being in command of the post. It then marched to Danville, where it joined the remainder of the army of Kentucky, and was in camp there until the latter part of January, 1853. While at Danville it participated in a forced march towards Lebanon for the purpose of checking the operations of the Confederate, John Morgan.

On January 26th it began the march to Louisville with the army of Kentucky and on February 1st embarked on steam transports at Louisville and moved thence via. the Ohio and Cumberland Rivers to Nashville, Tenn., arriving there on the 8th, where it went into camp on the Franklin Pike, three miles South of the city. On March 4th it moved to Franklin under orders to re-enforce Col. Coburn's brigade then in battle at Springhill, and on March 9th and 10th participated in the movement under Gens. Granger and Sheridan against Gen. VanDorn's army as far as Rutherford Creek near Columbia. On returning to Franklin it assisted in fortifying the place and on April 10th took part in the defense against VanDorn's attack, in which the Confederates were repulsed with considerable loss. Up to that time the 115th had suffered no casualties in battle, but by reason of exposure in its marching and scouting expeditions it had lost about 200 men, either dead or permanently disabled. While at Franklin, the army of Kentucky was changed into the Reserve Corps of the Department of the Cumberland, the 115th being assigned to the first brigade with its old associates, the 92nd and 96th Illinois, together with the 40th Ohio and the 84th Indiana, Col. Smith D. Atkins of the 92nd Illinois commanding the brigade which was a part of the 1st division under command of Brig. Gen. A. Baird, the corps being commanded by Maj. Gen. Granger.

On June 2nd the regiment marched to Triune, Tenn., where it participated in several scouting expeditions and skirmishes. On the 24th it moved with its brigade and divisions and the department in the Tullahoma campaign assisting in driving the Confederates from their strong positions in the hills near Shelbyville, through Duck River valley, and over the Cumberland Mts., and across the Tennessee River. On July 2nd the regiment occupied Wartrace, a month later moved to Tullahoma, and on September 5th entered upon the Chickamauga Campaign marching over the Cumberland Mts., via. Stevenson and Bridgport, Ala., and Chattanooga, Tenn., to Rossville, Ga.; the portion of the Reserve Corps taking part in the campaign of the 96th Illinois, the 2nd brigade commanded by Col. T.E. Champion of the 96th Illinois, the 2d Brigade being commanded by Col. Mitchell, both of the 1st division and the 2nd brigade of the 2nd division, commanded by Col. Daniel McCook; these three brigades composing for that campaign, what was known as Steedman's division of the Reserve Corps. About this time Brig. Gen. W.C. Whitaker resumed command of the brigade. On the 18th of September it participated, with its brigade, in the movements against Scott's Cavalry on the Ringold Road near the Chickamauga and the skirmish connected therewith. On the 19th of September it was with its brigade in the action near McAfee Church, suffering a loss of about 10 men. On September 20, 1863 it moved with its brigade, followed by Col. Mitchell's brigade, to the supoort of Maj. Gen. Thomas on Snodgrass Hill, which position was then about to be occupied by the rebel divisions of Generals Hindman and Bushrod Johnson. In the assault upon Snodgrass Hill the 115th was in the center of the front line of Whitaker's brigade and occupied that position throughout all the afternoon of the second day of the Battle of Chickamauga, retiring in obedience to orders about dusk in the evening, having suffered the loss of about one-half its number. The 115th was in the most exposed position of any regiment in that bloody battle and held its place against great odds, successfully meeting the repeated assaults of Anderson's Confederate brigade and when out of ammunition joined in the last bayonet charge that drove Preston's division from the summit of the ridge near the close of the battle. For its gallant conduct in that action it received special commendation of its commanding generals and was granted the honor of carrying the division colors on the following day, as a mark of distinction. Among its lost were Lt. Col. Wm. Kinman and Capt. S. Barlow Espy, killed, and Capt. S.M. Huckstep and Lt. David Reed, mortally wounded.

The regiment was in the seige of Chattanooga occupying a position at Moccasin Point and suffering the privations of the army and was in the engagements at Wauhatchie and other actions around Chattanooga. On the re-organization of the army of the Cumberland the 115th was attached to the 2nd brigade, 1st division, 4th army corps, department of the Cumberland, Gen. Whitaker commanding the brigade; Stanley the division; Granger the corps; and Thomas the department. After the Battle of Mission Ridge it remained in camp at Shell Mound, Tenn., until the latter part of January, 1864, Col. Moore being commander of the post. It then marched to Cleveland, Tenn., and in the latter part of February participated in the Dalton raid under Maj. Gen. Palmer, losing six men in the action about Tunnel Hill. After that it returned to camp near Cleveland and remained there until the opening of the Atlanta Campaign. The regiment led the chare on Tunnel Hill on May 7th and was in the actions in front of Rockey Faced Ridge, driving the enemy through Buzzard Roost Gap. It was with its brigade in the Battle of Resaca, Ga., in which the regiment stubbornly resisted a charge of overwhelming numbers and for which it was commended in general orders, its loss being one officer and 30 men killed and wounded. By general order there were inscribed on its regimental banner the names of all the principal engagements of the military division of the Mississippi which resulted in the fall of Atlanta, its loss, during the Atlanta Campaign being 100 men. When Gen. Hood threw himself on Sherman's rear in October, 1864 and was marching on Chattanooga Co. D of this regiment occupied a block house in Buzzard Roost Gap, and held in check Hood's army for 10 hours, fighting stubbornly; it refused to surrender the Gap till the block house was rendered untenable and nearly demolished by the enemy's artillery. One-third of the company of 41 men was killed and wounded; the remainder surrendered with the highest honors of war. For this special act of gallantry the company commander, Capt. Samuel Hymer of Schuyler county, was given the brevet rank of major, and later was awarded a gold medal by the war department. The balance of the regiment being then stationed at Tunnel Hill successfully defended itself against Hood's forces until ordered to retreat the following morning. When Sherman began his march to the sea and the 4th army Corps, under Gen. Stanley and the 23rd Corps under Gen. Scofield, started North, the 115th tore up the railroad track and hauled the rails away from Kingston, Ga., to Chattanooga. The regiment was then attached to Steedman's provisional division at Chattanooga and went with it to Nashville, Col. Moore commanding one of the brigades. The regiment then rejoined its old brigade and took part in the Battle of Nashville which resulted in the destruction of Bragg's old veteran army, then commanded by Gen. Hood. Soon after that Col. Moore succeeded to the command of the brigade, the regiment being with it in the pursuit of Hood to Northern Alabama, Hood having been driven with the remains of a broken army across the Tennessee, the regiment with the 4th army Corps marched to Huntsville, Ala., and went into camp on the 5th of January, 1865. On the 14th of March it started for East Tennessee, thence expecting to move by way of Lynchburg, Va., to assist in the capture of Richmond; but while in the vicinity of Greenville, Tenn., Richmond fell and the rebel Gen. Lee surrendered. The regiment then moved back with the 4th Army Corps and went into camp near Nashville, Tenn., and there remained till mustered out of the service, June 11, 1865. It arrived at Camp Butler, Ill., June 16, 1865, and received final pay and discharge June 28, 1865.

The brigade to which the 115th was attached, for nearly its entire service, without material alteration, was known throughout the department as the "Iron Brigade of Chickamauga."