Its first service in the field was in Missouri, where several months were spent on various expeditions, including hard marches, skirmishes, to which were added suffering from hardship and disease. Colonel Dewey died of erysipelas (a skin infection typically caused by group A beta-hemolytic streptococci) at Patterson, Missouri, on the 30th of November, and was succeeded by Kinsman, who was commissioned colonel on the 1st of December, 1862. The regiment was engaged in the hard march to Iron Mountain in February and soon after was sent down the Mississippi to Milliken ‘s Bend to join General Grant’s army in the campaign against Vicksburg, being assigned to the First Brigade of the division commanded by General Carr, where it remained drilling until the army marched to encompass the Confederate stronghold. Many of the gunboats and transports having run the batteries at Vicksburg and Grand Gulf, the army was concentrating at Bruinsburg. The Twenty-third joined in the march into the interior and was warmly engaged in the Battle of Port Gibson, where it did good service and lost thirty-three men. On the 17th of May, after several defeats, the Confederate army made a last stand on the banks of the Black River.

At this point a high bluff rises abruptly from the water’s edge on the west side. On the east approach there is an open level bottom about a mile in width surrounded by a deep muddy bayou, from ten to twenty feet wide. Along the bayou earthworks had been thrown up mounted with artillery and long lines of breastworks manned by infantry. Half a mile in the rear was a line of earth-works, both extending from the river above the bridge to the river below. This was a strong position, skillfully fortified by able engineers.

McClernand advanced to the attack with Carr ‘s Division on the right and Osterhaus on the left; General Lawler commanding the brigade in which was the Twenty-third Iowa on the extreme right of line. Several hours were occupied in skirmishing when Lawler ‘s Brigade was moved under cover of the river bank, from which he ordered an assault of the enemy’s works. The troops charged across the level bottom land, through the bayou, under a terrible fire from the Confederate earthworks, which covered the ground with the slain; but closing up the gaps they pressed on over the breastworks and captured eighteen pieces of artillery and 1,500 prisoners. Those of the enemy who escaped set fire to the bridge across the river to check pursuit. In this brilliant charge three hundred seventy-three brave men fell, most of whom belonged to the Twenty-first and Twenty-third Iowa regiments. Colonel Kinsman while leading his command was shot by two balls which passed through his body and he fell from his horse dead. Many of his officers and men were slain or mortally wounded, carrying great grief to scores of Iowa homes.

After the battle the regiment was placed in charge of several thousand prisoners, who were captured at Champion ‘s Hill and Black River Bridge, to be conveyed to Memphis. Returning, it was sent to Milliken ‘s Bend, where General Dennis was in command of about 1,500 men. They were encamped along the Mississippi between the river and the levee. Breastworks had been thrown up and rifle pits dug to protect the camp. The troops were mostly colored men who had recently enlisted and were under command of Colonels Lieb and Chamberlain. The Twenty-third Iowa, under Lieutenant-Colonel Glasgow, was now reduced by battle and sickness to about two hundred men fit for duty. On the 6th of June, Captain Anderson, with two companies of Illinois cavalry, and Colonel Lieb, with the Ninth Louisiana colored, made a reconnaissance on the Richmond road. They were attacked by Confederates when Colonel Lieb ‘s regiment opened fire, checking the advance. Colonel Lieb then returned to the Bend, where he was reinforced by the Iowa regiment. At three o’clock in the morning a large force of Confederates was discovered advancing in close column by divisions, with cavalry on the right. The little Union army in line waiting for the onset, withheld fire until the enemy was within short musket range, when it opened all along the line. The assailants wavered for a moment, but rallied and pushed on with the fierce " Rebel yell." The negroes fought bravely, but were greatly outnumbered, and finally forced back in a desperate hand-to-hand struggle in which clubbed muskets and bayonets were used in the deadly combat. Two gunboats now opened on the enemy, which was finally repulsed with heavy loss, leaving more than one hundred dead on the field. The Twenty-third Iowa took a conspicuous part in this battle under the leadership of Colonel Glasgow, fighting with unsurpassed courage throughout the long and desperate encounter, losing fifty men out of two hundred. Among the slain was Captain J.C. Brown, of Company I. This battle was notable as the first in which negro troops took a prominent part. The employment of colored men in the army had met with strong opposition from the time it was first proposed. But as the war continued public sentiment changed and many negro regiments were raised. The Battle of Milliken’s Bend demonstrated the fact that the colored men would not only fight bravely but in every respect made good soldiers. In this battle the Iowa regiment fought with the colored brigade, and side by side they won from General Grant warm commendation for their gallantry. After the battle the Twenty-third returned to its brigade in the army investing Vicksburg. Though weak in numbers it did good service in the various trying ordeals of the siege.

After Pemberton’s surrender the regiment was sent to reinforce Sherman’s army in operations about Jackson, and at the close of that campaign returned to Vicksburg. About the middle of August General Ord’s Corps was transferred to the Department of the Gulf, where for nearly a year the operations of the Twenty-third Iowa were intimately associated with the Twenty-second, as detailed in the history of that regiment. It was employed in Texas and the islands along the coast, then, returning to New Orleans in the spring of 1864, was sent to reinforce the defeated army of General Banks retreating down the Red River valley. It ascended the Mississippi with a command under General Fitz-Henry Warren and proceeded to Fort De Russey, and from there went into camp at the mouth of the Red River, joining General Banks’ army about the middle of May. Later in the season the regiment was placed in a brigade with the Twentieth Iowa, an Illinois and a Wisconsin regiment, and for a long time was employed in Arkansas without meeting the enemy. Early in 1865 the command returned to New Orleans to join the expedition then being fitted out for the last campaign of the war, that against Mobile. Colonel Glasgow was now in command of the brigade and Lieutenant-Colonel C. J. Clark commanded the regiment. In the hard marches, the siege and assaults of that brilliant campaign, the Twenty-third bore an honorable share. In storming the Spanish Fort it again met in combat the Twenty-third Alabama, which had been encountered at Port Gibson, where it was first under fire. Here one man was killed and twenty-five wounded.