Old Glory is flying once again in front of the U.S. embassy in Havana, Cuba. And at the flag-raising ceremony on Friday, Secretary of State John Kerry did everything he could to remind people of the history that brought it down 54 years ago. “For more than half a century,” he said, “U.S.-Cuban relations have been suspended in the amber of Cold War politics.”

The U.S. punditocracy, meanwhile, weighed in with predictable platitudes about the meaning of it all. Many complained that Cuban dissidents should have been invited to the embassy. The Washington Post called the State Department’s excuses for this failure “lame” and proclaimed, “The American flag is a powerful symbol of the country’s long and noble struggle to defend the values of freedom and democracy.”

Fair enough. But as we’ve learned in the course of this summer, flags can mean many things to many people. And if we want to have a better understanding of Cuba, now that it’s beginning to open up, we should remember that its troubled relations with the United States did not begin with Fidel Castro’s revolution in 1959 or even Teddy Roosevelt’s charge up San Juan Hill. We should understand that for many years the American flag—not the Confederate flag—was, for Cubans, the star-spangled banner of slavery.

Early in the 19th century, Great Britain, the United States, and most of the governments of Europe had passed laws banning the horrific slave trade between Africa and the Americas. The British, who finally emancipated the slaves in their colonies in 1833, moved not only to end their own previously extensive participation in the trade in humans, but to prevent others from carrying out that grim commerce as well. They deployed warships off the coasts of Africa and South America to stop, search, and seize suspected slavers, and they used gunship diplomacy more than once to impose their will on weaker nations.

But the United States had gone to war against Britain in 1812 to stop it from stopping and searching any American ships, and steadfastly refused to let the British anti-slaving fleet stop American-flag vessels. Instead, Washington deployed its own feeble squadron off the coast of Africa which did little to stop slavers and much to interfere with the British efforts to do so.

The main market for the slaves—tens of thousands of them every year—was the Spanish colony of Cuba, where it was more profitable to work them to death in the cane fields and then replace them with new, cheaply bought Africans, than it was to keep them healthy and alive. Technically, it was illegal to import them, but the law was ignored.

And, technically, trafficking in African slaves was illegal in the United States as well—it was supposed to be a hanging offense—but the New York shipbuilders and outfitters figured it was well worth the risk, and when cases were brought before the Southern courts they refused to indict.

Indeed, the pro-slavery faction in the United States had its own designs for Cuba: to buy it or conquer it and turn it into two new slave states, thus assuring control of the Senate and greater power in the House of Representatives. (Slaves had no rights as citizens or as human beings under the Constitution, but counted as three-fifths of a person for census purposes, thus hugely inflating the voting power of the states that held them.) More than a century before the Bay of Pigs fiasco, adventurers in the United States organized invasions of Cuba to “liberate” it from Spain in the interests of American slavery. Those, too, were fiascos.

It is difficult to conceive, today, just how gruesome was the trade carried out under that American banner of “freedom and democracy.” In the 1850s, Southern politicians known as “fire-eaters” were defending slavery—and the slave trade—as a moral good. They were pushing to reopen it between Africa and the United States. And at the epicenter of Southern radicalism, Charleston, many refused to acknowledge the grotesque inhumanity of the Cuban trade even when it stared them in the face.

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In late August 1858, the horror that the South did not want to imagine—a slave ship—was right there in Charleston harbor. Vomit and urine and feces and blood had seeped deep into the raw wood of the sunless slapped-together slave decks in the hold, staining them indelibly with filth. Cockroaches by the millions seethed among the boards, and clouds of fleas and gnats rose up from them.

The stench that came from this vessel wasn’t the smell of a ship full of cattle and horses, but that peculiar smell that surrounds humans, and only humans, who are very afraid and very sick, or dying, or dead. The water in Charleston Harbor was still and flat and thick as oil, and the air was stifling hot and heavy. The stinking vessel, a brig called the Echo, had been captured off the coast of Cuba a few days before.

Because it was the summer, the season of disease, many of Charleston’s better-off residents had left the city. For those who remained behind the spectacle of the Echo and its Africans was a disgusting but almost irresistible novelty. Because the transatlantic trade had been banned for 50 years, many had never beheld such a ship before. “You will see by this morning’s Mercury that we have a slaver in our harbor,” one distinguished Charlestonian wrote to a friend. “She has on board about 300 naked native negroes, 60 of them women. Every one of whom is in the family way. Everybody is talking about them. The yellow fever, the cables and every other subject have faded before this. There is really and truly an excitement among these cold, stolid Charlestonians.”

That the Echo had been captured at all was the result of a dawning awareness by the federal government of something that the British consul in Charleston, Robert Bunch, had been explaining to the foreign office in London for years: The fleets of slave ships flying the American flag, supported by money-men in New York, and incited and abetted by the fire-eaters like Robert Barnwell Rhett and Leonida Spratt, posed a growing threat to the authority of Washington and to the Union itself.

The slave traffic was growing fast. Something had to be done before the momentum became unstoppable. So, quietly and against stubborn bureaucratic resistance, President James Buchanan had American warships step up their anti-slaving patrols off the coast of Cuba as well as Africa. And the Echo was their first prize.

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The story of the Echo's capture, as told in the Charleston newspapers, began at dawn on Saturday, August 21, 1858, when the USS Dolphin pulled out of the Cuban port of Sagua-la-Grande, about 150 miles east of Havana. The Dolphin was a brig-of-war, and she had been on desultory anti-slaving missions off the coasts of Brazil and Africa from the time she’d first been commissioned more than 20 years before. With two square-rigged masts and six 32-pound guns she was, despite her age, quick and deadly by the standards of the time, but she’d seen little action.

Now, through the course of the morning, the Dolphin’s crew could spy far ahead of them the twin masts of another brig. They didn’t pay much attention. The stranger wasn’t a very big ship, and looked like the kind of boat used for coastal trade among the maze of islets and keys off the north shore of Cuba. But by around 1 in the afternoon, the Dolphin had pulled closer. Its commander, Lieutenant John Newland Maffitt, was about as shrewd a skipper as could be found in the U.S. Navy at the time, and he knew right away that this ship looked too light in the water for a coastal trader.

The Dolphin picked up speed. The strange brig started to take evasive action, hauling in some of its canvas and tacking hard to starboard with the kind of speed and efficiency that’s only possible when a ship is manned by a large and well-trained crew. Coastal traders didn’t normally have those kinds of men on board.

Maffitt decided to play a little game with his quarry. He ran up the British flag where the strange brig could see the colors clearly. The British could not board an American vessel without provoking an international incident, which is why so many slavers flew the Stars and Stripes whenever they thought they might be challenged by one of Her Majesty’s ships.

At first, the stranger showed no colors at all, but after hours giving chase, Maffitt fired a shot close under her stern. The stranger ran up the American flag. It had fallen into the trap. Maffitt took down the British ensign and ran up the American one. Still the stranger struggled to get away. Maffit fired again, this time under the ship’s bow. The stranger hauled down her sails, struck her American colors and threw them into the sea.

The armed boarding party from the Dolphin noticed the name of the ship, the Echo, was painted on a slab of wood nailed to the stern, but the original name, the Putnam, was still visible, a ghost image of white painted over in black.

“We found her a brig of 320 tons, filled with Africans,” one of the Dolphin's officers wrote to a friend later that week. “The appearance was most revolting; never can I forget it. There were 328 negroes crowded together between decks.” They were crammed in, half crouched and so closely packed that only the tops of their heads were clearly visible, and the stench was almost unbearable. “The poor wretches looked half starved, and some of them were mere skeletons,” wrote the officer. The sails and other fittings of the ship were American. Many of the crew were American. The captain, a Mr. Townsend, was from Boston.

Some 455 Africans had been taken on board the Echo near Kabinda on the African coast. More than 140 had perished during the weeks at sea and were thrown overboard. (“The shark of the Atlantic is still, as he has ever been, the partner of the slave trader,” wrote a British editorialist.) It took the U.S. Navy prize crew six days to sail the Echo to Charleston harbor, the most important American port within reach of the fetid vessel. By then, another eight captives had died. And they just kept dying.

Within hours of the Echo’s arrival, Charlestonians were debating what to do with the human cargo. There was no precedent for the arrival of Africans under these circumstances and there was no jail in the city that could hold them. In the Charleston Daily Courier the next morning, a correspondent writing under the name Curtius argued that the only “humane” thing to do would be to make slaves of them and train them to perform useful occupations in South Carolina.

In the event, federal officials decided to hold the Africans at a massive fort called Sumter that had been under construction for years near the entrance to the harbor and still was not quite finished. The Echo was brought in and anchored near the Customs House. But the town’s notables had to charter their own transportation to get a look at the Africans themselves:

“The gentlemen, representing a great variety of interests, were much gratified at the spectacle presented by these savages, who appeared in fine spirit and entertained their visitors with a display of their abilities in dancing and singing,” wrote the correspondent for the Charleston Mercury. The dancing was a grim ritual, in fact, that belonged to no particular culture in Africa, but to the trade in captives. Once a day at most, the people packed into the hold would be brought on deck to get some air. Those who were dead or dying were identified and thrown into the sea. Food was forced down the throats of those who refused to eat. Sometimes the captain, and rarely some of the other officers, took their pick of the women. And there on the shifting deck, someone would beat a drum “to dance the slaves,” to give them a little exercise. Those who refused were whipped. Then they were herded below again into their floating hell.

“The whole exhibition was exceedingly interesting and novel, in which the Negroes seemed to take great delight,” wrote the Mercury. “Very few are left in the hospital, and those manifest anxiety to get out. The ailments with which they are afflicted are readily yielding to medical treatment, and the general health of the gang has much improved.”

This was all a lie, of course. Yellow fever was raging in Charleston at the time and Fort Sumter was not immune. The Africans also “ate freely the shell-fish which collected around the fort, and died rapidly,” wrote a doctor assigned to care for them. “Their condition on leaving the brig Echo was painful and disgusting in the extreme. They had been huddled together closer than cattle, and slept at night in as close contact as spoons when packed together. Privation of every kind, coupled with disease, had reduced all of them to the merest skeletons, and to such a state of desuetude and debility that on entering the fort they could not so much as step over a small beam, one foot high, in the doorway, but were compelled to sit on it and balance themselves over. It is impossible for you to imagine their sad and distressed condition.”

The U.S. marshal at Charleston, who had been vocal in his support for reopening the trade with Africa, felt differently after watching over its victims for the three weeks the Echo’s people were held at Fort Sumter. “Thirty-five died while in my custody,” he wrote to a friend, “and at one time I supposed that one hundred would have fallen a sacrifice to the cruelties to which the poor creatures had been subjected on board the slaver. I wish that everyone in South Carolina who is in favor of the re-opening of the slave trade could have seen what I have been compelled to witness… It seems to me that I can never forget it.”

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For the Africans who had been kidnapped from their homes, loaded on the death ship en route to Cuba and rescued, after a fashion, by Lt. Maffitt, the tragedy continued. By the time those who survived Sumter reached Liberia aboard the U.S. warship Niagara, another 71 had died on board. The American agent charged with caring for them was furious at what he had seen and wrote to a friend in Britain that the Christian nations of the world had to unite to stop the trade. “Some new mode must also be introduced for the trial of those found on the slavers,” he wrote, “probably by trying them at once and swinging them up to the yard-arm.”

Consul Bunch, Her Majesty’s man in Charleston, was inclined to share the sentiment as it became obvious that South Carolina was not a place where justice could be had. Proceedings against the captain and crew of the Echo got under way in November and the whole affair took a “very remarkable” turn, Bunch reported in an official dispatch. The lead attorney for the defendants was none other than Leonidas Spratt, who “has made himself very conspicuous by his advocacy… of a revival of the Slave Trade.” And despite the overwhelming evidence against the captain and crew, the grand jury, in what Bunch called “a monstrous piece of absurdity,” refused to indict. In his confidential correspondence with London, Bunch was flabbergasted: “That the offense with which the men were charged was committed, and by them, no one professed for a moment to doubt. They were taken in the very act, and every witness was present who could affirm their guilt. There was, therefore, no loophole through which the grand jury could escape. And yet, such is the force of public sentiment, that they refused to allow a trial to take place.”

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In the first years of the American Civil War, Consul Bunch’s reporting from the previous decade about the American involvement with the slave trade from Africa to Cuba, and the Southern crusade to reopen the slave trade between Africa and the United States, had played a pivotal role in the Crown’s reluctance to support the Confederacy.

In early 1862, almost a year into the war, and still a year before the Emancipation Proclamation went into effect, President Abraham Lincoln proved his willingness to fight the African slave trade with two important measures:

First, he allowed the hanging in February 1862 of the American ship captain Nathaniel Gordon, from a prosperous family in Maine, who had transported thousands of Africans and “lost” hundreds of them thrown dead or dying into the waters between Congo and Cuba.

“Any man who, for paltry gain and stimulated only by avarice, can rob Africa of her children to sell them into interminable bondage, I will never pardon,” Lincoln declared. Gordon was the only slaver ever executed by the United States for his crimes against humanity.

Secondly, Lincoln agreed to a treaty allowing mutual search between British and American vessels, meaning slavers could no longer hide behind Old Glory.

It was only then—after the Union victory 150 years ago—that the Stars and Stripes could be said to be, in the words of this morning’s Washington Post, “a powerful symbol of the country’s long and noble struggle to defend the values of freedom and democracy.”

Adapted from Our Man in Charleston: Britain’s Secret Agent in the Civil War South. Copyright © 2015 by Crown. Reprinted by permission.