The 121st Ohio Infantry served in Mitchell's Brigade, Steedman's Division, Reserve Corps, on Thomas' right. Early on September 19, 1863, near Reed's Bridge, it moved under orders of the division commander to Red House Bridge. On September 20 at 11:30 A.M., it moved from McAfee's Church to support Thomas. At 2:00 P.M. the regiment charged, drove the enemy from this position, and held it until near sundown. It repulsed three persistent assaults, drove the enemy to the foot of the hill, and captured the colors of the 22nd Alabama. Near sundown it retired to the first hill in the rear, and about 6:30 P.M. withdrew with the army to Rossville. During the afternoon the regiment was on the right of the army. Of 235 engaged, 99 were killed, wounded, or missing. The regiment was organized at Deleware, Ohio, on September 11, 1862, for three years.