After June 14, Banks (North) stopped trying to take Port Hudson with men’s bodies and started taking it with engineering and time. This was the part the manuals were written for. Under his chief engineer, Captain John C. Palfrey (North), the Union army dug approach trenches (saps, zigzag ditches that crept the attackers’ line closer and closer to the works without exposing the diggers) and started three mines, tunnels driven under the earth toward Fort Desperate, the Priest Cap, and the Citadel, meant to be packed with powder and blown. The guns bombarded the fort day and night. And around all of it pressed the Louisiana summer, which turned out to be a weapon as deadly as any cannon.

Inside the ring, the garrison began to starve. They had been sealed in since May 22, with no way to resupply and a four-to-one army squeezing the perimeter tighter every week. They ate the beef first. Then, when the beef was gone, they ate the mules. Then the dogs and the rats. They brewed a kind of beer from sugar and molasses for want of anything else. By late June a private in those trenches was a man with bleeding gums and loosening teeth from scurvy, his belly cramping on a fistful of corn and a strip of mule, lying in a ditch that stank of latrine and rot and the sweet sick smell of gangrene from the field hospital, slapping at the mosquitoes that carried the malaria already shaking half his company with fever, listening to the Union guns work the line all night and counting how many men had simply slipped over the parapet in the dark to give themselves up rather than starve. A Confederate soldier’s diary recorded the descent plainly, that they had eaten the meat and the bread, then the beef, then the mules, then the dogs and the rats around them, and the diary, like the man, was running out of things to say. The National Park Service later borrowed a survivor’s phrase to name the ordeal: forty days and nights in the wilderness of death.

The siege was murderous to the besiegers too, in a quieter way. While the assaults killed in great visible bursts, disease and sunstroke killed steadily for weeks, felling somewhere on the order of 4,000 to 5,000 Union men over the campaign, a toll roughly comparable to all the battle casualties combined. The Mississippi summer did not care which uniform a man wore.

What finally ended it came not from the trenches but from upstream. On July 4, 1863, Vicksburg surrendered to Grant. The news came down the river by boat on July 7, when a Union vessel carried word into the lines that the great fortress upstream was gone; Gardner (South), stunned, asked to see the official documents before he would believe it. They were sent in. And with Vicksburg confirmed, the entire logic of Port Hudson collapsed in a single stroke. The fort had never been an end in itself. It existed to help hold the middle stretch of the river that Vicksburg’s fall had now lost completely. The Confederacy no longer controlled the Mississippi above Port Hudson, so there was nothing left for Port Hudson to be the southern lock of. Gardner was guarding a door whose wall had been torn down.

Gardner recognized his position for what it was: hopeless. There was no army coming to relieve him, no point left to defend, and a starving garrison that had already eaten its animals. On July 9, 1863, five days after Vicksburg, he surrendered Port Hudson. Roughly 6,340 men laid down their arms; most, around 5,935, were paroled, released on a signed oath not to take up arms again until they were formally exchanged for Union prisoners, which is what Civil War “parole” meant: not freed for good, but let go on their sworn word rather than marched to a prison camp. The exception was 405 officers, who were kept as prisoners and sent off to Memphis and New Orleans. The siege had lasted 48 continuous days, frequently cited as the longest true siege in American military history. And it had been decided, in the end, not by the bayonet that failed twice in the ravines but by the river and by hunger.