 It was designed to prevent Union forces from going upriver to Little Rock, and to disrupt Union movement on the Mississippi.On January 9–11 of 1863, Union forces conducted an amphibious assault on the fortress backed by ironclad gunboats as part of the Vicksburg Campaign.Union forces outnumbered the defenders (33,000 to 5,500) and won an easy victory in capturing the post, with most of the Confederate garrison surrendering.