With Port Hudson’s surrender on July 9, 1863, the Union held the entire Mississippi River, end to end, from its headwaters down to New Orleans and the Gulf. The last lock was open. For two years the river had been the spine of the war in the West, and now it ran unbroken in Union hands.

That did two enormous things. First, it split the Confederacy in two. The Trans-Mississippi (Texas, Arkansas, and western Louisiana, with their cattle, their grain, and their men) was now cut off from the eastern Confederacy by a river the South could no longer cross. The Red River artery that Port Hudson and Vicksburg had existed to protect was severed. The supplies that had been feeding the eastern armies stopped coming. Lincoln’s famous line about the Father of Waters (the Mississippi) going unvexed to the sea belongs to that larger opening of the river.

Second, it helped answer a question the country had been arguing about for two years: would Black men fight? On May 27, in the ravines below the bluff, the Louisiana Native Guard answered it with their bodies, and that answer kept echoing long after the guns stopped. Captain André Cailloux’s story did not end on the field. After the surrender, when his body was finally recovered from the ground where it had lain for those 47 days, New Orleans held him a public funeral on July 29, 1863 that drew thousands of Black New Orleanians into the streets, a vast procession through the city for a man who had been born enslaved and died a captain of the United States, leading free men against the system that had owned him. A priest, Father Claude Paschal Maistre, defied a Confederate-sympathizing archbishop to perform the rites. Cailloux became a national martyr-hero of Black military service and a recruiting symbol across the North; his funeral is remembered as a forerunner of the New Orleans tradition that would later become the “second line.” And the recruiting worked: the proof these men gave at Port Hudson, joined to Milliken’s Bend and that summer’s assault on Fort Wagner, accelerated the enlistment of the United States Colored Troops toward the nearly 200,000 Black men who would ultimately serve the Union, a great many of them lately enslaved, now in arms against the men who had held them.

Port Hudson carried a double meaning. As a battle it was a near-failure redeemed by an event 150 miles away: two slaughtering assaults that took nothing, a siege won by hunger and by Grant’s victory at Vicksburg rather than by Banks’s army. But as a moment in the war it was a hinge, the southern half of the act that opened the Mississippi and split the South, and one of the first proofs, paid for in the bodies of formerly enslaved men, that the people the war was being fought over would fight, and die, to free themselves.