Organized May 14, 1861, at Joliet, and mustered in June 13th. It left camp the next week for Alton, from whence it moved, July 6th, to Cape Girardeau, Mo, remaining there or in its vicinity seven months, during which it was engaged on several minor expeditions, and in some fighting. On February 2, 1862, — then in W. H. Wallace's Brigade, McClernand's Division, — it embarked for Fort Donelson, where it sustained a loss of 18 killed, 108 wounded, and 6 missing; total, 132; Lieutenant-Colonel William Erwin, an officer who had seen service in the Mexican war, was killed in this action, a round shot striking him in the breast; every man on the color-guard was either killed or wounded. At Shiloh the brigade was commanded by Colonel Marsh, the loss of the regiment amounting to 22 killed, 107 wounded, and 7 missing; total, 136. During the Vicksburg campaign in 1863, the Twentieth served in General John E. Smith's Brigade, Logan's Division, Seventeenth Corps. At the battle of Raymond, the regiment lost 17 killed — including Lieutenant-Colonel Evans Richards, — 68 wounded, and 1 missing; at Champion's Hill, 2 killed, 15 wounded, and 8 missing. It was stationed at Vicksburg, or in its vicinity, from July, 1863, to February, 1864, when it marched on the Meridian campaign. Returning from its veteran furlough, it joined Sherman's Army, June 8, 1864, while on the advance to Atlanta, the regiment being assigned to Force's (1st) Brigade, Leggett's (3d) Division, Seventeenth Corps. The Twentieth accompanied the army on its march through Georgia to the Sea, and on the campaign through the Carolinas, the brigade being then un(ler command of General Charles Ewing.