Early June 1862, Maj. Gen. David Hunter transported Horatio G. Wright's and Isaac I. Stevens's Union divisions under immediate direction of Brig. Gen. Henry Benham to James Island where they entrenched at Grimball's Landing near the southern flank of the Confederate defenses.
The stage was set for the Battle of Secessionville. On the June 15th, Gen. Benham laid plans for the Union forces to make an early morning surprise attack on the Secessionville fort, a "reconnaissance in force" as he so called it. He would use approximately 3500 of his troops to make a frontal assault before daybreak, attacking to 2 structured waves. During that day and throughout the night Col. Lamar had his troops working on shoring up the fort's defenses. Lamar finally dismissed his men at 4:00 A.M. By this time the union forces comprising the 8th Michigan, the 7th Connecticut, 28th Massachusetts and 79th New York Highlanders were on the move, supposedly at the double-quick, advancing on the fort. However, advancing through the darkness, the troops had to negotiate through 2 hedge rows and open cotton fields now knee-deep in weeds. This resulted in breaking up the initial Union lines and slowing the advance. As the field narrowed approaching the fort, the left side of the Union front was pushed into the marsh and got bogged down in the mud. It also compressed the Union center, slowing the advance such that the second wave ran into the first complicating the advance even more.
The Confederate advanced pickets were overrun about 5:00 A.M., this activity alerted the Confederates. Lamar mounted the parapet to observe the Union front about 700 yards and closing. He immediately dispatched couriers to Evans, who was 5 miles away at Fort Johnson with the reserves. As the defenders rushed to their stations, Lamar took personal command of the 8-inch-Columbiad. The Union lines were within 200 yards of the fort, Lamar order the Columbiad to fire; grapeshot, nails, iron chain and glass blasted from the cannon directly at the Union center, tearing a great hole through the Union lines. The Battle of Secessionville had commenced.
Lamar sharply ordered all gun commanders into action and moved the infantry into place firing volleys as they came onto line. The 8th Michigan on the Union right, was now clambering up the face of the fort followed by the 7th Conneticut troops and soon after the 28th Massachussetts The arrival of Confederate reinforcements drove the Union assault force back from the parapet when the Confederate defense was on the verge of collapse due to casualties on the critical gun crews. The Federals came under severe fire from 3 sides as they advanced up the fort walls, only to be ordered to fall back to regroup after suffering heavy casualties. Meanwhile, the 79th New York on the Union left actually mounted the fort's parapet and were engaged in hand-to-hand combat with the Confederates, when the Union artillery, located at the River's house, opened fire on that fort breaking up the Union attack and forcing the 79th to withdraw. As the first Union wave collapsed and retreated, they hampered the second wave from attacking.
The Federals fell back to the protection of the hedge rows to reform their lines. They were to make 2 more unsuccessful assaults before a general withdraw was ordered. While the main assault was taking place, the 3rd New Hampshire was attempting a flanking maneuver from the Union left. Their assault brought them to within several yards of the flank of the fort, but they could not make a full assault due to the water and mud that proved to be impassable. They proceeded to assault the fort from across the marsh, driving the Confederates from the parapets. The two 24-pounders that Lamar set up on his right flank were still silent, even though the new gun crews were on line. Lt. Col. Ellison Capers of the 24th South Carolina Infantry was sent up to the position to determine the problem.
The gun crew, although an artillery unit, had never been trained in firing a cannon before and did not know what to do. Capers sprang into action loading and firing the piece himself, while training the gun crew. Meanwhile, 250 men of the 4th Louisiana Battalion arrived from their encampment 2-1/2 miles away to sure up the Confederate flank and pour a decimating fire into the 3rd New Hampshire troops, causing the Union force to fall back. By 9:00 A.M., the battle was over.
Had the Federals captured Battery Lamar, they would have flanked the harbor defenses and might have forced the abandonment of Charleston by the Confederacy, cut the Atlantic Coast Line Railroad, and established a base for operations into the interior which might have ended the war 2 years sooner. Contrary to Hunter's orders, Benham had launched an unsuccessful frontal assault against Fort Lamar at Secessionville. Because Benham was said to have disobeyed orders, Hunter relieved him of command.