Organized at Cairo, September 18, 1861. In November it fought at Belmont, where it lost 10 killed, 70 wounded, and 4 missing. In February, 1862, it moved up the Tennessee River, and was present at the bombardment of Fort Henry; thence it marched over the hills to Fort Donelson, where it went into position in front of the enemy's works, amid winter's snow and storm. It was engaged in the assault, during which the regiment evinced a remarkable steadiness under fire, changing front to rear on tenth company in the face of severe musketry, over uneven ground and in tangled brush. Colonel Logan, who still commanded the Thirty-first, was seriously wounded in this action, and Lieutenant-Colonel John H. White was killed; the total loss was 31 killed, 117 wounded, and 28 missing. During the Vicksburg campaign, the Thirty-first served in J. E. Smith's Brigade, Logan's Division, Seventeenth Corps; it lost at Raymond, 1 killed and 6 wounded; at Champion's Hill, 5 killed and 18 wounded; and on May 22d, in the grand assault on Vicksburg, 3 killed and 21 wounded, including Lieutenant-Colonel John D. Rees, who was mortally wounded by a hand-grenade while mounting the parapet. It also sustained severe losses in the trenches during the siege, several being killed in the fight over the crater at the Mine Explosion at Fort Hill, May 25th. The regiment encountered its hardest fighting and greatest percentage of loss on July 22, 1864, at the battle of Atlanta; it was then in Leggett's (3d) Division, Seventeenth Corps.