David Eltis and David Richardson wrote this article for the New York Times.

Several years ago the historian David Brion Davis wrote an intriguing article in which he asked, and answered, a counterfactual question: What if the Confederacy had won recognition from Britain in 1862 and survived the war? His rather frightening answer was that the three great centers of slavery in the Americas — the American South, Cuba and Brazil — plus the smaller plantation economy of Dutch Suriname, would not have abolished slavery when they did.

That the Civil War and the subsequent Thirteenth Amendment ended slavery in the United States is self-evident. What’s less well known is that the Civil War also had immense significance for the ending of slavery elsewhere, especially in the Americas. Indeed, few people recognize that it took a war to finally bring the brutal transatlantic slave trade to a close. In all likelihood, without a Union victory, slavery would have remained a central institution underpinning global economic growth until possibly the present day.

It’s true that only a small share — about 4 percent — of the total slaves carried off from Africa landed on the North American mainland. And an even smaller share of those destined for slavery in the rest of the Americas completed their voyage in vessels flying the United States flag, or set sail from mainland ports.

Yet small as the United States role was, there is no doubt that the federal government effectively protected transatlantic slave traders in the half-century before 1861 and that the outbreak of the Civil War just as effectively removed that protection. Indeed, thanks to America’s role, almost one-quarter of the total transatlantic slave trade occurred after the government banned American participation in the slave trade in 1807.

How was it possible for such a minor player to have such a large impact? The answer turns on the nature of the nascent system of international law that had emerged by the early 19th century, and the fact that the transatlantic slave trade was perhaps the most thoroughly multinational business of the early modern era.

For one thing, Denmark, the United States and Britain, the first nations to take action against the trade, might have banned their own citizens from participating and forbid the entry of slaves into their own territories, but without negotiating international treaties, they could do nothing to stop foreign nationals carrying on the slave trade elsewhere — including the high seas.

After 1807, the British constructed an elaborate, costly but ultimately ineffective network of treaties that allowed their cruisers to stop suspected slave vessels flying the flags of other nations. But it didn’t cover all countries, and in these years slave ships sailed under the colors of Mexico, Russia, Sardinia and Argentina, among many others, solely because these flags prevented British intervention. And as long as a single nation allowed its flag to be used in this way, and as long as Brazil and Cuba remained open to new arrivals from Africa, the transatlantic slave trade would continue.

Moreover, France and the United States never allowed the British to stop and search their own merchant vessels. Instead, they undertook to patrol the Atlantic themselves. The French at one stage assigned more than 40 warships to anti-slave trade duties. The American fleet, on the other hand, never exceeded six warships and for years at a time the country deployed none at all. And neither nation sought permission to stop and search suspected slave ships operating under another flag. As a consequence, the British navy accounted for almost 80 percent of the nearly 2,000 slave vessels detained in Atlantic waters in the era of suppression; the United States detained just 68.

Why such a half-hearted effort? The answer is simple. American administrations were often stocked with Southerners in key positions like secretary of state, secretary of the navy and president, and they refused to take serious action against the foreign slave trade. Thus they tacitly allowed the Stars and Stripes to be used as a cover. In the absence of a treaty the British were reluctant to interfere with American shipping; only American naval ships could stop this practice, and even when they acted officers would usually detain a ship only if slaves were on board (thus ships heading to Africa, even if they were obviously slavers, were let go).

But an unwillingness to enforce the law was just part of the story. American shipbuilders also sent a steady stream of fast sailing vessels into the slave trade after 1830. The premium on speed had partly to do with voyage mortality risks and the high-value nature of the cargo — by the 1860s, 500 slaves could fetch close to half a million dollars in Cuba. The design of the slave vessel changed radically between 1800 and the mid-1830s, and the duration of the middle passage to Cuba declined by one third between the 1790s and 1850s. Many of these fast slavers were yachts or clippers made in the United States: Baltimore, in particular, developed a reputation for constructing fast sailing vessels. In all, about one-third of the slave ships sailing after 1810 were built in American ports.

Second, the use of the United States flag by slave traders escalated after 1835. In the early years of suppression, the Cuban trade was conducted under the Spanish flag, the traffic to the French Americas under the French flag and the traffic to Brazil under the Portuguese and Brazilian flags. Only a few of these vessels were American, with an American master and crew. But this situation began to change in 1835 when Great Britain and Spain signed a treaty that for the first time specified a range of equipment that could provide grounds for detention. In other words, slavers without slaves on board could be detained. The immediate effect was that slave traders abandoned the Spanish flag, and began to register their vessels as Portuguese, though a growing number sought registration under American colors.

The British introduced other measures in 1839 and 1845 that extended the so-called equipment clause to Brazilian and Portuguese slave traders, thus increasing reliance on the American flag even further. For a few years a weird set of flags appeared on the African coast, with vessels frequently carrying more than one set of papers, each to be used as required. Many ships flew no flag at all.

The use of the American flag ended only after the Civil War began. In 1862, with Southern politicians finally gone from national politics, the United States at last signed a treaty with the British providing for mutual right of search on the high seas, an equipment clause and joint Anglo-American joint courts (called Courts of Mixed Commission) for adjudicating detentions. The fact that those courts never heard a single case detracts not at all from their impact.

True, the slave trade continued for another five years, but at decreasing annual volumes. More importantly, there were only three recorded voyages under the United States flag after the 1862 convention — and, we should add, after the execution that same year of Nathaniel Gordon, the only individual to suffer the full penalty of the 1820 Act that made slave trading a capital offense. Compare this to 123 such voyages documented in the five years preceding the 1862 treaty. Secession, and a Union victory in the war that precluded any renewed trading under the Confederate aegis, made all the difference.

For the original report go to http://opinionator.blogs.nytimes.com/2011/01/13/the-transatlantic-slave-trade-and-the-civil-war/