By the start of 1865, the Confederacy, the slaveholding republic the Southern states had broken away to build, was running out of doors to the outside world. The United States Navy had spent three and a half years tightening a blockade around the Southern coast, sealing one port after another, until almost nothing could get in or out by sea. One port still worked: Wilmington, North Carolina, on the Cape Fear River. Fast, low-slung ships called blockade runners still slipped past the warships there in the dark, hauling in the rifles, powder, shoes, and food that kept the Confederate armies in the field. Wilmington was the last lung the Confederacy had left, and the thing that kept it breathing was a fort of sand down at the river’s mouth.

That fort was Fort Fisher, and it was a monster. It was not a stone castle but a vast L-shaped earthwork, walls of packed sand and turf piled high and thick, with a long land face guarding the neck of the peninsula and a sea face fronting the Atlantic. Between its big guns ran a row of huge mounds called traverses, the heaped earth between gun positions, designed to box in any shellburst and shield the crews. Sand absorbs cannonballs the way stone never can, swallowing the shot and closing back up. Men called it the Gibraltar of the South, and on paper it deserved the name.

Lieutenant General Ulysses S. Grant, running the whole Union war effort, understood exactly what the fort meant. While he held Lee pinned in the trenches at Petersburg, Virginia, Wilmington was still feeding the army he was trying to starve. Close Fort Fisher and the river closed with it, and the last supplies stopped reaching Lee. It was, in the cold arithmetic of the war’s final winter, one of the last doors left to shut.

The Union had already tried once, and made a fiasco of it. In December 1864 a joint expedition came down to take the fort: the army under Major General Benjamin Butler (North) and the fleet under Rear Admiral David D. Porter (North). Butler had a scheme he was sure would crack the place open without a fight. He had an old steamer, the USS Louisiana, packed with about 215 tons of gunpowder, towed in close at night and blown up, the theory being that the blast would flatten the fort’s walls and stun its garrison. The ship went off in the dark of December 24 with an enormous roar, and did essentially nothing. It had been anchored too far out, and Fort Fisher’s sleepy garrison barely noticed.

After a heavy naval bombardment, Butler landed troops, looked at the fort, decided it could not be taken, and re-embarked his men without a serious assault, sailing back to Virginia on Christmas Day. Grant was furious. He relieved Butler of command and resolved to send the expedition straight back, this time under a soldier who would actually go in.

The soldier he chose was Major General Alfred H. Terry (North), handed a provisional corps of about 9,600 men drawn from the Army of the James. Terry and Porter would return to the same beach barely two weeks later, with the same fleet and the same fort, and finish what Butler would not.