Union forces had landed on the island in early July 1863. The first assualt on the fortifications, garrisoned by around 1,800 men, was by three brigades on July 11 and failed. A second force was gathered by Major General Quincy Gillmore and dispatched against the fort in the early hours of July 18.
The approach to the fort was constricted to a 60 m wide strip of beach. After a bombardment from both land and sea the Union infantry moved in. The assault force was headed by the 54th Massachusetts Volunteer Infantry and included five other brigades, around 5,000 men in total. Unfortunately for the assault force, the prior bombardment failed to seriously damage the fighting power of the fort. Consequently, the Union infantry suffered a great deal of casualties in the rush towards the fort.
Once there, the fighting proved intense. Three brigades managed to occupy a portion of the walls, but they were forced to withdraw after a hour of fierce fighting where almost every officer was killed. The Union forces had taken around 1,600 casualties and the Confederate garrison under 200.
However, the battle proved to be a political victory for the Union since the valour of the 54th against the hopeless odds proved the worth of black soldiers. It spurred additional recruitment which gave the Union forces a further numerical advantage in troops.
The fort was then besieged after the unsucessful assault. After almost 60 days of heavy shelling, it was abandoned by Confederate forces on September 7, 1863.