Reflecting a burst of post–Cold War optimism about the ability of international institutions to foster human rights and the rule of law, the 1990s were a golden age for the creation of international courts. The decade began with the United Nations Security Council’s authorization of ad hoc international criminal tribunals for the former Yugoslavia and Rwanda, and culminated in 1998 with the treaty creating the permanent International Criminal Court (ICC).

But that optimism soon waned. It was expensive to operate the tribunals, and the initial results were disappointing. They lacked police and had to rely on the cooperation of national governments to bring defendants into custody; for the ad hoc tribunals’ first few years, only a few lower-level defendants were brought to trial. The prosecutions seemed to have little deterrent effect. One of the worst massacres of the Bosnian war took place at Srebrenica in the summer of 1995, while judges in the Hague were already hearing evidence on other crimes. Critics said the tribunals provided cover to powerful countries unwilling to intervene to prevent atrocities. The Rwanda tribunal operated at a painstakingly slow pace, and skeptics wondered why the worst genocidaires were held in a relatively luxurious international prison, exempt from the death penalty, when thousands of lower-level offenders languished in local jails facing harsher justice and some of their victims lived in worse circumstances still.

There was another burst of optimism in 2001, when former Yugoslav president Slobodan Miloevic was turned over for trial. But the accused quickly transformed the proceedings into a slow-moving farce, conducting his own defense for the benefit of television viewers back in Belgrade as each ethnic group in the Balkans complained that the court was biased against them. Finally, almost spitefully, Milosevic died of a heart attack in 2006, before the trial could be finished. Observers pronounced the case a colossal failure.

Still, there was much to admire about both ad hoc tribunals, which had been quietly prosecuting many other mid- and high-level offenders with increasing efficiency. The Security Council’s tribunal for Yugoslavia completed trials for well over one hundred accused. While the Rawanda tribunal heard fewer cases, it convicted the former Rwandan prime minister and numerous other high-level officials. The trials unearthed the truth about many horrendous crimes and established a historical record. Many victims felt vindicated by their day in court, with more than 3,500 witnesses testifying at the Yugoslavia tribunal alone. The trials brought closure to some communities, allowing them to put the past behind them and move on. The tribunals’ decisions contributed to the development of international law, elaborating and applying legal principles that had before existed only in the abstract. These successes, however, received relatively little public attention.

Given such a mixed record it is fair to ask, Can international courts do any good, or are they a waste of effort at best and a dangerous threat to national sovereignty at worst? To answer that question it is instructive to turn to dusty colonial courtrooms in 19th-century Freetown, Havana, and Rio de Janeiro.

For better or worse, everyone thinks of international courts as a 20th-century invention. But in the 19th century, international courts located in these cities were responsible for enforcing treaties designed to eliminate the transatlantic slave trade. Although they had civil, rather than criminal, jurisdiction, their powers were impressive. The courts heard more than 600 cases and freed almost 80,000 slaves found aboard illegal slave-trading vessels. These are extraordinary numbers on the scale of international justice. But these courts are virtually unknown, even to most international lawyers and human rights activists.

The effective suppression of the transatlantic slave trade is perhaps the greatest success story in the history of human rights law. A centuries-old trade, involving millions of people and generating enormous profit, was extinguished within a single human lifespan. Some have argued that the slave trade died a natural death, with the rise of industrial capitalism. But the best historical evidence suggests otherwise, that it was intentionally wiped out by people and nations who had come to believe that trading in slaves was an unconscionable violation of human rights, and who were willing to pursue its abolition even at great personal cost. International treaties—enforced at times by international courts and at other times by national courts—were a lynchpin of the effort.

The history of this feat suggests a more complex relationship between political power and moral ideas than either hard-nosed foreign-policy realists or idealistic human rights advocates today acknowledge. And, importantly, it provides grounds for cautious optimism about the contemporary possibility of promoting human rights through international law, at least in the long term.

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At the beginning of the 19th century, slavery was the cornerstone of the Atlantic economy. Between 1800 and 1810, more than a half million slaves arrived to replenish the human capital of the New World’s plantation economies. At the same time, under the influence of religious-revival movements, along with the secular writings of Enlightenment philosophers, a growing number of people across different countries had come to view slavery as morally wrong. Upon introducing an early and unsuccessful bill to ban the slave trade 1776, one member of the British Parliament argued that the “Slave-trade was contrary to the laws of God, and the rights of man.” Using tools of civic activism that would be familiar to modern human rights activists, the early abolitionists relied on petitions, pamphlets, sugar boycotts, and speaking tours to generate popular support for their cause. Fashionable young women in London and Philadelphia coveted jasper cameos from the famous Wedgwood ceramics house bearing the image of a kneeling African man in chains, with the inscription “Am I Not a Man and A Brother?”

By 1807, the argument that slavery violated each human being’s natural right to the fruit of his or her own labor had gained political traction, and in that year Britain passed landmark legislation banning participation in the slave trade by its subjects. (It would be decades before slavery itself was outlawed in the British Empire. Among the many striking features of British and American attitudes towards slavery was their ability to separate the issue of the slave trade from the continued existence of slavery itself, a distinction drawn by other nations as well.) Similarly, President Thomas Jefferson’s message to the U.S. Congress in 1806 supported legislation opposing the slave trade because it would “withdraw the citizens of the U.S. from all further participation in those violations of human rights which have been so long continued on the unoffending inhabitants of Africa.” A federal law banning the slave trade took effect in the United States in 1808, the first year that such a ban was permitted under the U.S. Constitution.

The slave trade was an international enterprise, however, and no country alone could end it. Slave merchants could avoid the ban on trading under British and American flags by flying French, Spanish or Portuguese flags instead. Moreover, countries that had banned the trade were placed at a competitive disadvantage. British ship owners were deprived of the impressive profits of slave trading and British plantations in the Caribbean were starved of new workers, while rival French colonies continued to import slave labor. North American plantations, where slave mortality rates were lower, felt the pinch less strongly, but with sectional divisions over slavery already creeping into national politics the United States was lackadaisical about the use of the American flag by slave traders.

And so a combination of idealism and material interest led Britain to become for a time the world’s self-appointed anti-slavery police. During the Napoleonic Wars (1804-1815), Britain took advantage of a feature of international law (then called the “law of nations”) that during wartime permitted the search of vessels on the high seas to determine whether they were enemy ships in disguise or carrying contraband in violation of the laws of neutrality. Such ships could be condemned as prizes of war, with the profits enriching the pockets of the naval officers who made the capture as well as the national treasury.

In 1808, a British warship invoked the laws of neutrality as grounds for searching the Amedie, sailing under the American flag. The British crew found a cargo of slaves onboard (something they could probably detect even before boarding, from the notorious stench of human misery that was known to carry miles downwind from slave ships), and the British courts upheld the ship’s seizure and condemnation. Although the customary law of nations did not prohibit the slave trade, the appeals court concluded, the trade was so contrary to natural law and justice that it was presumptively illegal in the absence of evidence that a particular nation allowed it. Since the United States itself had banned the trade, the American ship owners could have no legal claim that their rights were being violated by the British seizure. Between 1807 and 1815, British courts relied on this reasoning to condemn dozens of American, French, Spanish, and Portuguese slave ships. In each case the slaves onboard were freed, and the ship and its remaining cargo were auctioned off, with some of the proceeds going to the British government and the rest as “prize money” to the crew of the capturing ship.

Although Britain emerged from victory over France and its allies as the dominant maritime superpower, its right under the law of nations to search vessels on the high seas ended with the conclusion of the war. In peacetime, no nation had the legal right to interfere with another’s ships, unless there was suspicion of piracy. (While some countries would eventually declare the slave trade a form of piracy, it did not yet have that status under the general law of nations.) The same British courts that had upheld the wartime seizures of slave ships would soon begin to invalidate such captures.

At the same time, the British government was under increasing domestic pressure to make suppression of the slave trade central to its post-war foreign policy. In the summer of 1814, three-quarters of a million British citizens (out of a population of twelve million) signed petitions denouncing the peace treaty with France for not including a clause immediately banning the slave trade under the French flag. It was taken as a sign of tremendous public concern that such a large number of people would sign petitions in such a short period of time, especially in the preindustrial age when communications were slower. Faced with intense domestic political pressure, Britain had two options: to use its maritime dominance to act unilaterally and illegally against the slave trade, or to try to persuade other nations to enter into treaties and cooperate in suppressing the traffic through legal means. It chose the latter course.

Following hard lobbying by the British, the European nations participating in the Congress of Vienna signed an agreement in 1815 that condemned the slave trade as “repugnant to the principles of humanity and universal morality,” but it established no schedule for its abolition and provided no enforcement mechanism. This was progress, but nothing revolutionary. The previous year, a similar aspirational clause had been included in the Treaty of Ghent, which settled the War of 1812 between the United States and Great Britain.

Continuing bilateral negotiations between Britain and Spain, Portugal, and the Netherlands led to more impressive results in 1817. Through a combination of threats, bribery with outright cash payments, and moral persuasion, Britain induced these countries to sign treaties banning the slave trade that also provided means of enforcing the ban through a mutual right to search one another’s ships on the high seas. Even more significantly, these treaties provided for the creation of international courts to implement the ban. Slave vessels would be brought by the capturing nation before an international “mixed court” for trial. Each country would appoint a judge, and in the event of disagreement, an arbitrator from one of the countries would be selected by lottery to cast the deciding vote. For example, a captured slave vessel sailing under the Spanish flag would be brought in front of a court consisting of a Spanish and an English judge, with a Spanish and an English arbitrator waiting in the wings to break any ties. If the ship was found to be illegally engaged in the slave trade, the slaves would be freed and the ship would be auctioned off, with the proceeds divided among the governments and the crew of the ship that had made the capture. The slave ship’s crew members could be sent to their own nation’s courts for criminal trial.

The great legal innovation was that these courts were permanent and prospective. Previous international treaties had provided for the retrospective resolution of disputes between nations by arbitration commissions appointed specially for that task; for example, the 1794 Jay Treaty between Britain and the United States established a commission to settle lingering boundary disputes and property claims from the Revolutionary War. But an international court with jurisdiction over cases that might arise in the future was a novelty. Indeed, it would be more than a century before any other permanent court was created. That institution, the Permanent Court of International Justice, was created as part of the League of Nations in 1921 and is mistakenly regarded by most international lawyers as the first permanent international court.

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The original anti-slavery courts were set up in Freetown, Havana, Rio de Janeiro, and Surinam. Later treaties added courts in Capetown, Loanda, and even New York. The court in Freetown heard by far the most cases (more than 500), both because of its location on the coast of Africa, where most of the captures occurred, and because of the local British government’s favorable attitude toward the court. The Havana and Rio courts heard a few dozen cases each, while those in the other cities heard a handful of cases at most. Although the courts operated from the 1820s to the 1860s, they heard most of their cases in the 1830s and 1840s. During the peak years, as many as one in every five or six ships involved in the transatlantic slave trade ended up in the international courts, and the vast majority of them were condemned.

Almost 80,000 slaves were freed by the courts, and an unknown number of other souls were spared slavery because some ships were captured before they were able to load their human cargo onboard. Still other ships may have been deterred from sailing because the threat of capture increased insurance costs and reduced the profiility of slave trading. Some of those emancipated by the courts ended up in apprenticeships that were virtually equivalent to slavery, especially in Cuba and Brazil. But the vast majority of those freed were located in Sierra Leone where they had a much greater chance of retaining their freedom, though few were able to find the way home to their villages and families.

The British navy captured almost all the slave vessels brought to trial, since no other country was willing to devote so many ships to the effort: in the 1830s and ’40s, between a sixth and a quarter of the British navy’s ships were involved in anti-slavery patrols. Although service in the West African Squadron was a hardship posting, with uncomfortable living conditions and the dangers of tropical disease, the promise of prize money allured. In addition, many naval officers were—or became—true believers in the abolitionist cause after encounters with slave ships.

Captain Joseph Denman, for example, was the son of a prominent abolitionist who was for a time the Lord Chief Justice of England. As a young lieutenant in 1834, Joseph Denman was given command of a prize crew aboard the captured slave ship Maria da Gloria. The ship was first taken to the Anglo-Brazilian court at Rio, which declined jurisdiction on the grounds that the ship was Portuguese, not Brazilian. Denman was forced to sail the unfortunate vessel back across the Atlantic to the Anglo-Portuguese court in Sierra Leone. During the voyage, 78 of the roughly 400 slaves on board died, and the rest were severely debilitated. Denman later testified before parliament about this voyage, explaining that in his four months aboard the ship he “witnessed the most dreadful sufferings that human beings can endure.” Denman captured many slave vessels during his subsequent service on the African coast, which lasted over the next few years, and also led a not-entirely-authorized but successful expedition to destroy slave warehouses onshore.

Denman’s commitment was not unique. In addition to the naval officers who were the arms and legs of the suppression effort, many British colonial and consular officials provided its eyes and ears, sending reports to London about the intended routes of suspected slave ships. Many judges, too, who served on the courts became true believers after inspecting overcrowded slave ships firsthand. Dedication to the suppression effort was necessary: even relatively high pay and handsome pensions could not compensate for the high risk of death that accompanied service in a tropical climate. A number of the judges died at their posts, particularly in Sierra Leone. The British were generally prompt about appointing replacements, but other countries were not; for long stretches the courts were international in name only, with non-British seats remaining vacant. The judges faced other challenges: as with modern international courts, the judges on the anti-slavery courts struggled with differences in language and national legal customs, and had to improvise hybrid legal procedures.

There were loopholes in the treaties too—clauses about where slave ships were sailing and whether they actually had slaves on board at the time of capture—that frustrated naval captains and judges by allowing violators to escape the courts’ jurisdiction. And lax enforcement of the laws by local officials in Cuba and Brazil (the biggest slave importers after the U.S. government effectively suppressed the North American trade), who often turned a blind eye to the illegal sale of slaves, impeded suppression efforts. British judges and consular officials in Havana and Rio complained bitterly about them to London. Finally, the non-participation of major powers France and the United States in the initial treaties allowed slave traders to elude capture by switching from the Spanish, Portuguese, or Brazilian flag to the Stars and Stripes or le tricolore. British naval captains would send to London lists of suspected slave ships flying the French or American flags, and the British Foreign Office would dutifully forward them to the French and American ambassadors with angry remonstrances.

For decades British officials worked to close these loopholes. Its diplomats persuaded Spain and other nations to sign treaty amendments that broadened the scope of the courts’ jurisdiction. Britain also sometimes resorted to forceful unilateral action to bring partner nations back into the fold. In 1839, for example, an act of parliament authorized the seizure and condemnation of Portuguese slave ships in British courts. (The use of domestic British courts in this fashion presages the modern exercise of universal jurisdiction over human rights violators like former Chilean strongman Augusto Pinochet.) The measure was legally justified by a creative interpretation of the existing treaties, though the legal argument was a bit of a stretch. The Portuguese government protested and war seemed possible. But within three years Portugal had signed a new treaty with fewer loopholes and engaged in more robust enforcement. Trade under the Portuguese flag dropped sharply.

In a similar crisis, after Brazil refused to renew an expired treaty with Britain authorizing the courts in the late 1840s, the British navy began seizing Brazilian slave ships and bringing them for trial in British courts. Britain argued that a separate treaty with Brazil declaring the slave trade to be piracy allowed their domestic courts to exercise jurisdiction over piracy. Again, the legal argument was “creative.” The crisis reached a head when the British seized several ships in Brazil’s territorial waters and exchanged a few shots with a Brazilian fort.

Neither country wanted full-fledged war, and the only way Brazil could save face was to take its own action against the slave trade. The Brazilian legislature debated such measures in 1851, with one leader noting that “Brazil is now the only nation which has not acquiesced in this [anti-slave] system” created by the network of international treaties. “Can we resist the torrent? I think not.” While Britain’s show of force was the catalyst, Brazil’s policy changed when it began to perceive itself as lying outside the international order of civilized states. Moreover, having for thirty years been partner to a legal scheme that condemned the slave trade as immoral, it became increasingly difficult for Brazil to justify continued tolerance of the trade both to domestic abolitionist groups and to other nations. And over time, domestic attitudes toward the slave trade had simply changed. The Brazilian parliament passed new anti-slave-trade legislation, executive officials began to enforce it, and by 1853 the number of slaves imported into Brazil essentially dropped to zero.

There were other holdouts. Despite British pressure, France never agreed to the system of mixed courts. It did, in the 1830s, sign a treaty with Britain providing for mutual rights of search, though captured slave ships were to be taken to their own national courts for trial. Moreover, French warships on anti-slavery patrols helped deter the use of the French flag in the trade.

For its part the United States, already wary of international legal entanglements, was only willing to sign aspirational agreements with no enforcement provisions. Concerns about both sovereignty and freedom of the seas prevented it from joining the mixed-courts regime or from ratifying a mutual search treaty. Both on its own initiative and pursuant to treaty obligations, however, the United States at times maintained its own anti-slavery squadron off the coast of Africa. Some 103 illegal slave ships were captured and brought for trial in U.S. courts between 1837 and 1862. The United States had long since stopped slave imports, and so most of these ships were headed to either Cuba or Brazil.

By the 1860s, only slave trade to Cuba remained, and it was carried out largely under the American flag. In the midst of the Civil War, in 1862, President Abraham Lincoln’s administration signed an anti-slave-trade treaty with Britain that provided for mutual rights of search and the establishment of mixed courts in New York and Capetown. Henry Brougham, who as a young man had helped push the 1807 anti-slave-trade legislation through the British Parliament, reportedly called the new Anglo-American treaty the most important development in the half-century war against the slave trade. With the network of treaties complete, no flag was safe from the British anti-slavery patrol. Within Cuba attitudes toward the trade had also changed over time, and local officials finally began enforcing laws against slave traders found onshore or in Cuban ports. The last remaining trade route withered, and within a few years the transatlantic slave trade had been extinguished.

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The suppression of the transatlantic slave trade, and the role of law and the courts in its undoing, is a remarkable story about the complex relationship between political power and moral ideas. Most people who study international relations are realists of one sort or another, and in conventional realist wisdom states act to support intangible and idealistic goals like human rights only when those actions are relatively costless: whatever their rhetoric, nations choose money and power over their ideals.

Suppressing the slave trade was, however, extremely costly. By one modern estimate, Britain’s effort cost an average of nearly two percent of its national annual income for each year between 1807 and 1867, and the direct costs of its yearly efforts between 1816 and 1862 were roughly equal to the annual profits it had received from the trade between 1761 and 1807. Not only was it costly, but it required a very long national attention span. The resources expended on suppression required the continued commitment of successive governments over a period of decades.

Britain’s campaign to end the slave trade is thus a puzzle for students of international relations. Some have suggested that Britain acted out of self-interest because its own plantation colonies in the West Indies were already in decline and it wanted to eliminate competition from other nations’ more prosperous colonies. But the evidence suggests that the decline of British plantations did not begin until well after the trade was banned. Contemporaneously, nations such as France and the United States skeptically supposed that Britain’s humanitarian agenda was a cover for the more cynical aim of gaining control of the oceans. But such control appears to have been neither the principal intent nor the effect of Britain’s anti-slave-trade policies. Moreover, the private letters of many British officials during this period reflect a genuine moral and humanitarian zeal for the abolition effort, stemming sometimes from religious conviction, sometimes from secular notions of individual rights, justice, and humanity.

To be sure, vanity, self-righteousness, paternalism, and the desire to compensate for less noble colonial endeavors may have had something to do with British efforts. And Britain might not have worked to abolish the slave trade if doing so had been truly devastating to its economic interests. But on the whole, the weight of the evidence suggests that Britain pursued the abolition of the slave trade because most people in Britain thought it was the right thing to do.

At first, people in other countries did not agree or did not care. But their governments were eventually induced by hook or by crook to sign treaties acknowledging that the slave trade was unjust and inhumane. In time, due to a variety of factors, people in those countries came to believe that it was so. When that happened, their governments began enforcing the treaties. Other nations joined the treaties, until a practice that had once been universally accepted was universally condemned as a violation of human rights and international law. It sounds like a fairy tale, but in this case it happens to be true. The story of the suppression of the slave trade suggests that the idea of human rights has in the past been powerful enough to generate international cooperation to overcome entrenched interests, and that it may prove powerful enough to do so again.

At the same time proponents of human rights are too often dismissive of the importance of national state power. They support the creation of institutions like the ICC and assume that the court’s power will spring from its legitimacy, from the rightness of the laws it is supposed to enforce and its adherence to lawful and transparent procedures. Those things are undoubtedly important. But international law and institutions may have the greatest chance of success when backed by powerful nations willing to use their harder powers to support the soft power of international law. The success of the anti-slave-trade treaty regime depended in no small part on the commitment of Britain’s military and economic power. At critical moments, Britain was forced to deploy its hard powers, as well as its domestic laws and courts, to pressure other nations into a sustained fight against the slave trade. Modern international courts, such as the Rwandan and Yugoslavian tribunals, have also depended on the assistance of powerful states. And modern domestic courts have been important partners in the enforcement of international human rights law, using universal jurisdiction to fill in gaps in the work of international courts.

When powerful nations back international institutions, however, there is a risk they will use those institutions to their own advantage or, in being perceived as doing so, they might undermine the institutions’ legitimacy. That risk escalates when the nations in question exempt themselves from the very laws and institutions they impose on others. The Bush administration, for example, now seems to view international criminal courts as tolerable and even useful, as long as they only prosecute deposed dictators or warlords from failed states in Africa. The same administration claims to be promoting human rights in Iraq and Afghanistan, while it denies human rights to prisoners it has captured in those countries. It claims to be building democracies, while it undermines constitutional democracy at home through sweeping claims of executive power to override the law.

Hypocrisy has a cost; the rhetoric of the rule of law has little chance of succeeding when actions show contempt for law. American neoconservatives have gone furthest astray not in their belief that promoting of democracy and human rights is a worthy and attainable goal of U.S. foreign policy, but in their support for conduct that contradicts that goal.

Nineteenth-century Britain encountered plenty of skepticism over its motives in the anti-slave-trade campaign, the historical equivalent of contemporary accusations that the United States went to war in Iraq for oil, power, or defense-contractor profits. And the accusations were understandable: at the same time Britain was campaigning nobly for the liberation of African slaves, it was engaging in less savory colonial endeavors in India, Africa, and the West Indies. Sometimes, it interpreted international law rather creatively to advance its goals. But in terms of the slave trade, at least, Britain subjected itself to the legal rules it sought to impose on others. It gave France, America, Spain, Portugal, Brazil, and a host of other countries the right to search ships flying the Union Jack to make sure there were no slaves on board. Wisely, Britain saw that the advantages of using international law and institutions to suppress the slave trade outweighed the constraints on its sovereignty that the legal regime imposed. Britain’s willingness to subject itself to the international laws and institutions it had created gave its actions a legitimacy that today’s United States will lack so long as we treat international law as a set of rules for weaker states.

We must be cautious about extrapolating lessons from a range of historical circumstances. Still, the suppression of the slave trade is suggestive for the present. If the people of the United States believe that promoting democracy and human rights around the world is a worthy goal, we should at least remember the experience of the first international courts and the role that international law played in the suppression of the slave trade. Power has its place, but so does principle.