The United States transatlantic slave trade wasn’t supposed to last all the way to the Civil War. And it wasn’t supposed to be a profit center for America’s abolitionist North.

But even after Congress banned U.S. participation in the Atlantic slave trade in 1807 and declared it piracy in 1820—a crime carrying the death penalty—a robust illegal trade continued. American shipowners, merchants, seamen and corrupt officials, based largely in New York City, collaborated with foreign allies to continue shipping captive Africans via the Middle Passage all the way into the 1860s. The practice not only inflicted terrible suffering on enslaved Africans; it also deepened the national divide over the institution of slavery, a rift that helped lead to the bloodiest conflict in U.S. history.

The United States was not alone in outlawing the slave trade—all major slaving nations abolished it by 1836—but that didn’t end anti-Black racism or the profit motive. Global demand for sugar, coffee and cotton grew enormously in the 1800s, and planters in the Americas sought captive laborers to help them meet it. Traffickers themselves had big incentives to defy international abolition: Profits for slave traders rose to 90 percent, up tenfold from a century earlier.

The United States played a key role in this illegal traffic from the start. Slave traders brought some 8,000 captives to the American South in the decades after the 1807 ban, including hundreds just before the Civil War. Among the last captives brought to U.S. soil was Oluale Kossola (renamed Cudjo Lewis), a young Yoruba man who sailed aboard the Clotilda, the last slave ship to arrive in the United States in 1860; before his death in 1935, he gave a powerful series of interviews to anthropologist Zora Neale Hurston, chronicling the trauma of being captured, sold and shipped to a foreign place to live—and labor—in bondage.

But the greater contribution by far was the use of American vessels as slave ships. Slave traders loved fast vessels such as the Baltimore Clipper, which could outrun slave patrols, including the American and much larger British squadrons. The U.S. government also refused to allow other nations to intercept vessels owned by American citizens. As a result, slave traders across the Atlantic basin flocked to the U.S. flag, often using American citizens in Rio de Janeiro and Havana as straw buyers. In the end, half a million captives came to Brazil and Cuba in American slave ships in the years after the transatlantic trade was banned.

In the 1850s, a particularly brazen group of slave traders known as the Portuguese Company set up headquarters in the booming metropolis of New York City. Led by Manoel Cunha Reis, a Portuguese trader who had trafficked enslaved Africans in Brazil and Angola, this group bought up ships secondhand in Manhattan’s vast shipping market. Then they worked with American seamen, ships outfitters and corrupt officials like marshal Isaiah Rynders to get ships on their way to Africa on what they pretended were legal voyages.

New York became internationally notorious in these years, especially as the slave trade to Cuba, the main destination for captives, boomed. Almost all slavers after 1850—around 500—were American, and most had New York connections. British newspapers called New York “the greatest slave mart in the world.”

British authorities were among the fiercest opponents of the slave trade. Since the American government largely turned a blind eye to the traffic and remained unwilling to let the Royal Navy intercept American slavers, the British consul in New York, Sir Edward Archibald, took matters into his own hands by hiring a spy. His named was Emilio Sanchez, and he is one of the great unknown abolitionists of American history.

Born in Cuba, Sanchez had immigrated to the United States and become a shipowner and merchant in New York. After an entanglement with members of the Portuguese Company ended badly, he was eager for revenge. He interviewed with Archibald and signed on for 400 pounds a month plus bonuses for each voyage terminated due to his information.

Sanchez put his knowledge of Manhattan’s docks to good use. For three and a half years, he spied on the slave traders, watching their movements and the departures of their ships. He struck up conversation with captains, sailors and outfitters, probing them for information. What was the name of the vessel? Its owner? When would it leave New York? He wrote it all down—often in cypher, a safety precaution—for Consul Archibald.

Archibald sent Sanchez’s intel across the Atlantic Ocean to London and to British cruisers off the African coast. Often it arrived with British ships before slavers had arrived from U.S. ports. Armed with information that the vessels’ true owners were not U.S. citizens, and therefore not entitled to fly the U.S. flag, the Royal Navy struck. In total, Sanchez’s intel ended 30 slaving voyages and prevented some 20,000 captives from enduring the Middle Passage.

As the slave trade developed in New York, the nation became increasingly divided over slavery. During a fierce dispute over the institution in Kansas, a new party emerged: the Republicans, led by Abraham Lincoln and William Seward. Unlike the ruling Democratic Party, which supported slavery and had little interest in crushing the illegal slave trade, the Republicans spoke out powerfully against both.

Instead of training their guns on New York, though, the Republicans reserved their fiercest criticism for the South. It was politically convenient: A handful of vocal southern radicals like Leonidas Spratt of South Carolina sought to reopen the slave trade to their shores in the mid 1850s. Republicans, and many northerners, were appalled.

William Seward, a New Yorker, lasered in on the issue, arguing that the “restoration of the African slave trade” was a southern priority, a scheme that would seed slavery on the expanding western frontier. He made the issue a key part of his “Irrepressible Conflict” speech in 1858. Abraham Lincoln did the same in his famous “House Divided” speech the same year.

Southerners and their northern allies responded by charging Lincoln and the North with hypocrisy and telling them to deal with the traffic “under their own eyes.” But when a few vessels, including the Clotilda, arrived in the Deep South between 1858 and 1860, Republican criticisms gained extra potency. Blocking “southern reopening” became part of Lincoln’s platform in the 1860 presidential election. Like the Kansas Nebraska Act, Dred Scott and John Brown’s raid, the illegal slave trade became a hot burning coal in the fire of the sectional crisis.

When the nation split after Lincoln’s election, it was actually the Confederacy that took the first steps against the slave trade. Recognizing that the issue divided Confederates at a time when they needed unity more than ever, leading political figures banned the traffic entirely in the Confederate Constitution in 1861.

Lincoln also moved against the trade by permitting the British to search U.S. vessels through the Lyons-Seward Treaty and refusing to commute the death sentence for a slaving captain Nathaniel Gordon, who became the only American executed under the 1820 law. Spooked, the Portuguese took flight.

By 1863, the American slave trade had finally ceased.

John Harris is McDonald-Boswell Chair in History at Erskine College and the author of The Last Slave Ships: New York and the End of the Middle Passage. Follow him on Twitter: @drjohnaeharris.

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