Gillmore was assigned to replace Maj.
Gen. Ormsby M. Mitchel in charge of the X Corps after that officer's death from yellow fever.
In addition, Gillmore commanded the Department of the South, consisting of North and South Carolina, Georgia and Florida, with headquarters at Hilton Head, from June 12, 1863, to May 1, 1864.
Under his direction, the army constructed two earthen forts in coastal South Carolina—Fort Mitchel and Fort Holbrook, located in the Spanish Wells area near Hilton Head Island.

He then turned his attention against Charleston, South Carolina.
Initially successful in an attack on the southern end of Morris Island on July 10, Gillmore had enough confidence to assault Fort Wagner on the north end of the island.
The following day he launched the first attack on Fort Wagner which was defeated.
He assembled a larger assault force and with the assistance of John A. Dahlgren's naval fleet planned a second attack.
On July 18, 1863 Gillmore's troops were repulsed with heavy losses in the Second Battle of Fort Wagner.
Gillmore's divisional commander, General Truman Seymour was wounded and two brigade commanders, George Crockett Strong and Haldimand S. Putnam were killed in the attack.
Gillmore was assigned to replace Maj. Gen. Ormsby M. Mitchel in charge of the X Corps after that officer's death from yellow fever. In addition, Gillmore commanded the Department of the South, consisting of North and South Carolina, Georgia and Florida, with headquarters at Hilton Head, from June 12, 1863, to May 1, 1864. Under his direction, the army constructed two earthen forts in coastal South Carolina—Fort Mitchel and Fort Holbrook, located in the Spanish Wells area near Hilton Head Island.
He then turned his attention against Charleston, South Carolina. Initially successful in an attack on the southern end of Morris Island on July 10, Gillmore had enough confidence to assault Fort Wagner on the north end of the island. The following day he launched the first attack on Fort Wagner which was defeated. He assembled a larger assault force and with the assistance of John A. Dahlgren's naval fleet planned a second attack. On July 18, 1863 Gillmore's troops were repulsed with heavy losses in the Second Battle of Fort Wagner. Gillmore's divisional commander, General Truman Seymour was wounded and two brigade commanders, George Crockett Strong and Haldimand S. Putnam were killed in the attack.
Quincy Adams Gillmore