IX Corps (Ninth Corps) was a corps of the Union Army during the American Civil War that distinguished itself in combat in multiple theaters: the Carolinas, Virginia, Kentucky, Tennessee, and Mississippi.

Formation, Second Bull Run, and Antietam
On February 5, 1863, Sedgwick was succeeded by Maj. Gen. William F. "Baldy" Smith, and on February 12 the corps was ordered to Newport News, where it encamped for a month. General Smith's stay with the corps was of short duration, for he was succeeded in the following month by Maj. Gen. John G. Parke. While at Newport News, Getty's (3rd) Division was detached and ordered to Suffolk, Virginia, where it was subsequently incorporated into the VII Corps. It never rejoined its old command although, in 1864, one of its regiments, the 4th Rhode Island, was restored to the IX Corps.
In the meantime, General Burnside had been assigned to the command of the Department of Ohio, a district that included Kentucky and eastern Tennessee. He obtained permission for the transfer of his old corps to this field of operations, and on March 19, 1863, General Parke was ordered to proceed there with his two remaining divisions, Willcox's and Sturgis's. Just prior to the departure from Virginia, General Sturgis was relieved, and Maj. Gen. Robert B. Potter was assigned to the command of the 2nd Division. The IX Corps was stationed in Kentucky for two months, during which it served as an army of occupation. In June it was ordered to the support of Ulysses S. Grant, who was then besieging Vicksburg, and proceeding there promptly, it participated in the investment of the city, although not under fire. Upon the surrender of Vicksburg, Parke's two divisions joined the main army in its movement on Jackson, Mississippi, and became engaged in the fighting there, with a loss of 34 killed, 229 wounded, and 28 missing; total, 291. The 1st Division was then under command of Maj. Gen. Thomas Welsh, General Willcox having been assigned to duty in Indiana. The Vicksburg Campaign had been a costly one for the Corps, not due to battle casualties, but to disease. Among those who succumbed to the deadly malaria of the Vicksburg camps was General Welsh, who, soon after, went home to die.
The corps left Mississippi in August, 1863, and returned to Kentucky, where, after a short rest, it joined in Burnside's advance into East Tennessee, a movement that had already commenced. The two divisions were now reduced to about 6,000 men. General Parke having been made chief of staff of the Army of the Ohio, General Potter succeeded to the command of the corps, with Generals John F. Hartranft and Edward Ferrero in command of the two divisions.
Ferrero's Division had a sharp little fight at Blue Springs, Tennessee, October 10, 1863, and the whole corps was engaged, November 16, at Campbell's Station. This was followed by the occupation of Knoxville and the gallant defense against James Longstreet's besieging forces, terminating December 4, in the defeat and withdrawal of the enemy.

Western Theater
General Willcox resumed command of the corps on January 17, 1864, relieving General Potter; on January 26, Parke relieved Willcox, who then took command of the 2nd Division.
General Burnside was again assigned to duty as commander of his old corps, which was ordered to Annapolis, Maryland, for reorganization. In April, the corps was composed of the four divisions of Thomas G. Stevenson, Potter, Willcox, and Ferrero, the latter division being composed wholly of African-American troops. The corps numbered 19,331 present for duty, with 42 pieces of field artillery; but this number was soon increased, the morning reports of May 10 showing a strength of 32,708. In addition to the four divisions, with their two batteries each, there was a brigade of reserve artillery of six batteries and a provisional brigade of heavy artillerymen and dismounted cavalry. In all, there were 42 regiments of infantry, and 14 batteries of light artillery. Ferrero's Colored Division had never been under fire, while many of the white regiments in the corps were newly organized, or had served previously on garrison duty only. In the ranks of the old regiments were many recruits and conscripts.
In the Overland Campaign, Ulysses S. Grant launched a major offensive against Robert E. Lee starting in May, 1864. His command in northern Virginia consisted of the Army of the Potomac, under Maj. Gen. George G. Meade, and the IX Corps operating separately, reporting directly to Grant. Burnside technically outranked Meade and objected to receiving commands from a junior officer. By the time of the Battle of North Anna, however, Grant acknowledged that this arrangement was ineffective and he convinced Burnside, along with General Parke, his chief-of-staff, to waive the question of their superiority of rank over Meade. On May 25, the IX Corps was assigned to the Army of the Potomac.
In the Battle of the Wilderness the corps lost 240 killed, 1,232 wounded, 168 missing; total, 1,640; and, at Spotsylvania, 486 killed, 2,119 wounded, 469 missing; total, 3,146; the heaviest loss at Spotsylvania Court House occurring in the action of May 12. The corps assaulted the right, eastern, flank of the "Mule Shoe" salient. General Stevenson was killed at Spotsylvania on May 10, and Maj. Gen. Thomas L. Crittenden, formerly commander of the XXI Corps, was assigned to the command of Stevenson's (1st) Division.

Overland Campaign
On June 9, at Cold Harbor, General Crittenden was relieved at his own request, and Brig. Gen. James H. Ledlie was placed in command of the 1st Division. In the first assault on Petersburg, June 17, the corps made a brilliant attack, Potter's Division gaining possession of the works; unfortunately, the division was obliged to relinquish its foothold for want of proper support. The corps was engaged in a similar attempt on the following day, the losses in Potter's and Willcox's Divisions being unusually severe in proportion to the number engaged. Losses were 497 killed, 3,232 wounded, and 262 missing; total, 2,991. The dead included Brig. Gen. James St. Clair Morton, chief engineer of the corps.
The enemy's works proving too strong for assault, the army entrenched itself in preparation for the ten-month siege that followed. On June 19, Ferrero's (4th) Division of black troops rejoined the corps, having been absent during the whole of the previous campaign, engaged on duty at the rear. Ferrero's men were now placed in the trenches with the other three divisions. The part of the line occupied by the IX Corps was very near the enemy's works, and an incessant firing was kept up during the siege, resulting in a daily loss of men, killed or wounded. While there was a comparative quiet in front of the other corps positions, the men of the IX Corps were subjected to the terrible strain of a constant watchfulness and deadly exposure. The enemy seemed to be excited to an undue activity by the presence of Ferrero's Colored Division.
The IX Corps was prominently connected with the siege because of its role in the infamous Battle of the Crater, in which a lengthy mine shaft was dug from its line to under the Confederate works. This mine, which was excavated by the coal miners of 48th Pennsylvania, Potter's Division, was successfully exploded with four tons of gunpowder, resulting in an enormous crater, but the assault that followed was a failure. Just before the assault, Grant and Meade directed Burnside to replace Ferrero's black division with Ledlie's white division, fearful of the political consequences of using black troops at the forefront of a major assault. But Ledlie's troops were not trained for the assault and Ledlie himself remained in the rear area, inebriated. The assault was a fiasco, in which Union soldiers charged into the crater, becoming trapped there, easy targets for the Confederates along the crater's rim. Ferrero's black regiments went into action in the rear and fought well, but had no chance for success. The losses in the IX Corps at the crater were 473 killed, 1,646 wounded, 1,356 missing; total, 3,475. Immediately after this engagement, General Ledlie was relieved from command of the 1st Division, and Brig. Gen. Julius White, of the XXIII Corps, was assigned to Ledlie's place.
On August 13, 1864, General Burnside was granted a leave of absence; he never rejoined the corps, but was succeeded by General Parke, who remained in command until the close of the war. At the battle of the Weldon Railroad, August 19–21, 1864, the three divisions of White, Potter, and Willcox were engaged with considerable loss, although the three combined numbered less than 6,000 muskets; casualties were 60 killed and 315 wounded. By this time the divisions had become so reduced in numbers that a reorganization of the corps became necessary, and so the regiments in White's Division were transferred to the divisions of Potter and Willcox. Under this arrangement Willcox's Division was numbered as the 1st; Potter's, as the 2nd; Ferrero's black troops were designated as the 3rd Division. But in December, Ferrero's Division was permanently detached, and most of his regiments were transferred to the newly organized XXV Corps, which was composed entirely of black troops. General Ferrero himself was assigned to a provisional command at Bermuda Hundred.

1865