Every movie needs a climactic moment. Romantic comedies have a wedding; sports films have the big game where everything is on the line. As the credits roll, the movie asks of us one final suspension of disbelief—we must be willing to accept that the story is over. We are not encouraged to examine too closely how, or whether, the blissful couple will make it to their third anniversary, or where those underdog high school basketball players will be in a few years. The movie makes a bargain with us: don't ask too much about the future, and you'll get a genuine, if momentary, happy ending.

History has such moments, too, and the vote for the abolition of the slave trade in the British Empire was certainly one. On March 25, 1807, after decades of agitation and advocacy by William Wilberforce and his allies, Parliament passed the Act for the Abolition of the Slave Trade. Slavery would not be entirely banned for another quarter-century, but it was no longer legal for British subjects to traffic in persons.

There remained only the small matter of enforcement. A host of newly minted outlaws were plying their trade far from Westminster, along three thousand miles of West African coast. What exactly would compel these captains and traders to give up their lucrative business?

In 1819, after it became clear that the slave trade was continuing in spite of Parliament's disapproval, the first seven ships of the Royal Navy's newly created Preventive Squadron sailed for West Africa. The most powerful navy in the world, which had successfully blockaded most of the navies of Europe over the previous decade, was embarking on what we might now call a "peacekeeping mission." The London Times offered a valedictory as they sailed: "Soon, we shall hear no more of the slave trade." Surely now the credits could roll.

This is where Siân Rees' Sweet Water and Bitter begins. It is quite a sequel. Firepower, it turns out, was not the only thing wanting. Most European powers were not as quick as Britain—if quick is the right word—to pass their own acts of abolition. The United States was decades from settling the question of slavery in its own territory and skittish toward any British interference with its affairs or its merchantmen, whatever their cargo. Slavers on the high seas were perfectly legitimate as long as they did not bear the flag of a country that had joined Britain in outlawing the trade. Rees documents a dizzying variety of gambits used to outwit the Preventive Squadron: carrying more than one set of flags, keeping several sets of logs, or arriving in a destination like Havana under one name and departing under another, with the help of local painters and accommodating harbormasters.

So British officers might board a ship—not an easy decision to take, since if its flag were not British, and its cargo turned out to be legitimate, this was an act of war—discover its hold packed with slaves and its crew speaking English, and yet be presented with a logbook that confirmed that it was, say, a Brazilian vessel, and have to disembark with grimly polite apologies. Along the West African coast, even more elaborate cat-and-mouse games flourished—it took two decades to amend the relevant treaties simply so that the Squadron could interdict ships that were fitted out for slave-trading but happened to have no actual slaves on board.

Morale did not improve when the navy had to chase down traders who disappeared into the tangled river deltas. Between the malarial miasma of the interior, the frequent plagues that swept through the ports, and the stifling tedium of months stationed off the coast, grave illness, insanity, and death were all plausible outcomes for anyone posted to the Squadron. Even when a slave ship was captured and delivered to port—a task that required two or three junior British officers to command a surly crew across leagues of ocean—the case would then enter molasses-slow proceedings cobbled together under international law, always assuming that judges representing more recalcitrant countries like Spain deigned to make themselves available for the proceedings. Captains were personally liable if they acted in violation of the treaties, so that a Squadron officer could board a slaver, free hundreds of Africans otherwise bound for the New World, and end up months or years later having to return the ship, and pay additional damages, to its original owner because of a legal technicality.

Rees' tale of frustrating setbacks punctuated by small, savory victories makes it clear that March 25, 1807 was at best "the end of the beginning" of abolition. Today, trafficking (a phrase that includes both domestic and transnational exploitation) is already illegal in nearly every jurisdiction—that battle, at least, has been won. But the actual defeat of trafficking, to judge by the Preventive Squadron's story, requires an odd combination of tedium and valor. Rees focuses mostly on the latter, but offstage in her account are countless patient diplomatic missions, reams of carefully documented legal motions, polite but insistent murmuring at the bench of one judge after another—and without all this terribly unexciting bureaucratic work the slavers would continue to sail through loopholes in the law as wide as the lanes of the sea. At the same time, generations had to be willing not just to die heroic deaths in naval combat, but to sink into the grip of malaria or worse. "Over the sixty years of its existence, the Preventive Squadron freed about a hundred and sixty thousand slaves and lost seventeen thousand British seamen to death by disease, battle or accident." Most of those 17,000 did not die valiantly in the usual sense of the word—they died miserably, though of course no more miserably than the countless victims of the Middle Passage. They were heroes only in this terrible and true sense: they gave their lives out of duty, in a cause they may or may not have chosen.

And this leads to a further lesson from the six decades of the Preventive Squadron. The only structures that can truly stop criminal networks are states. For only states can summon and sustain the vast diplomatic and bureaucratic apparatus that goes by the misleadingly concise name "the rule of law," and only states can order enough men and women into mortal danger, armed with instruments of force, to confront those who are making a healthy profit on violence. The Preventive Squadron was ultimately successful in its mission, but that success required one sixth of the total capacity of the Royal Navy at the height of the effort. In our time the best comparison may be the "War on Drugs"—an effort whose meager successes and human and financial toll remind us just how hard it is to disrupt trade that is flexible and profitable, even if illegal.

Americans, especially evangelical Americans, are heirs of a commendable tradition that valorizes the noble individual (like Wilberforce) and the voluntary association (like his Clapham Sect), and regularly one hears of a new evangelical non-governmental organization dedicated to "stopping trafficking." But trafficking will be stopped, if and when it is stopped, by a dauntingly vast army of patient bureaucrats and by officers of the peace who put themselves in harm's way and are willing to mete out harm when necessary. The best NGOS understand this and are dedicated to strengthening judicial and law-enforcement systems; but every time I find another Twitter user whose 160-character bio includes the phrase "abolitionist" I wonder if they know how boring, how costly, and how institutional that work must be to be finally done.

To be sure, only moral conviction and a sense of divine calling could have sustained the British interdiction effort, in both its diplomatic and naval facets, through to the end. (Worth pondering, too, is the role of empire, that much-maligned word, and whether Britain would ever have undertaken its largely thankless preventive efforts without it.) Christian voluntary efforts provided an essential moral backbone for the 19th-century's victory over trafficking, and may they do so again in whatever victories are won in our own century. The good news is that trafficking in persons today is probably less profitable than slavery was in the 19th century or drugs are in the 21st, and for the most part it is conducted by more socially marginal actors (the Russian mafia may be an unsettling exception). We can reasonably hope to see modern-day slavery dramatically diminished in our lifetime, but we need the sobriety that Sweet Water and Bitter provides. Without diplomatic finesse, legal expertise, and front-line valor—and the commitment to both transforming and maintaining national and international institutions—the battle will not be won. In fact, it will not even be begun.

Andy Crouch is the author of Culture Making: Recovering Our Creative Calling (IVP) and executive producer of Christianity Today's This Is Our City.

Copyright © 2012 Books & Culture. Click for reprint information.