Union efforts to retake Charleston Harbor began on April 7, 1863, when Rear Admiral Samuel Francis Du Pont, commander of the South Atlantic Blockading Squadron, led the ironclad frigate New Ironsides, the tower ironclad Keokuk, and the monitors Weehawken, Pasaic, Montauk, Patapsco, Nantucket, Catskill, and Nahant in an attack on the harbor's defenses (The 1863 Battle of Fort Sumter was the largest deployment of monitors in action up to that time).

The attack was unsuccessful, the Union's best ship, USS New Ironsides never effectively engaged, and the ironclads fired only 154 rounds, while receiving 2,209 from the Confederate defenders.

[2] Due to damage received in the attack, the USS Keokuk sank the next day, 1,400 yards (1,300 m) off the southern tip of Morris Island.

Over the next month, working at night to avoid the attention of the Federal squadron, the Confederates salvaged Keokuk's two eleven-inch Dahlgren guns.

A workforce of just under 500 slaves, under the supervision of Confederate army engineers, were filling casemates with sand, protecting the gorge wall with sandbags, and building new traverse,[4] blindages,[5] and bombproofs.

Dahlgren refused to place his sailors and marines under the command of an army officer, so two flotillas set out towards Fort Sumter that night.

"[7] This underestimation of the Confederate forces on Dahlgren's part may explain why he was hostile to a joint operation wishing to reserve the credit for the victory to the navy.

After the unsuccessful boat assault, the bombardment recommenced and proceeded with varying degree of intensity, doing more damage to Fort Sumter until the end of the war.

The last Confederate commander, Major Thomas A. Huguenin, a graduate from The Citadel, The Military College of South Carolina, never surrendered Fort Sumter, but General William T. Sherman's advance through South Carolina finally forced the Confederates to evacuate Charleston on February 17, 1865, and abandon Fort Sumter.