This is a really complicated and far reaching question, but I will attempt to answer it from the perspective of an American historian.

One of the problems with saying that slavery is a near continuous element through history or culture is you have to ask yourself exactly what you mean by slavery. Today, most people have a pretty clear idea in their head of what slavery means. It probably involves African descended people being whipped by white masters all while working in Antebellum American cotton fields. These people are probably enslaved from birth to death, and their children are automatically slaves as well. This kind of slavery was not the kind of slavery that existed for all time, so drawing a continuous like through history isn't really right. This is a specific kind of slavery called Chattel Slavery, which treated the slave as actual property which their owner could basically use or dispose of as they saw fit.

But what else is slavery? If you travel back to the 14th and 15th centuries you might find that what is slavery and what is not slavery isn't so clear. Most societies in this time still considered the women and children of his household to be almost like his property. Coverture, a legal construction in English common law allowed all of a wife's property to be treated as if it was the husbands property. They were a single legal entity under the law. The same went for his children. Society and common law also expected a woman to follow the orders of her husband, work in his household, birth him children, and submit to him sexually. Her husband could beat her, within some socially acceptable level of violence, and the concept of spousal rape was completely alien. A woman might not be able to travel without her husbands permission. Is this slavery?

At this same time, and even through the late 1700s there are also people who sell their labor or are forced to labor as a punishment. We usually call these Indentured Servants. But a close look at indentured servitude contracts reveals how punishing they could be. The contracts often stipulate many years of hard labor, and often carries stiff penalties for refusing to follow your master's orders which extend the contracts years more. You can imagine these were abused. Masters often beat their servants, subjected them to sexual abuses and rape, and gave them little to no money or sustenance. Some might say, well they entered into the contract willingly, but what does willing in this case mean? A young woman had no real rights of her own since she was a member of her father's household as discussed above, so he was free to sell her as a servant if he felt she did not have any marriage prospects. Young men in large cities like 16th century London suffered from rampant work shortages and often found themselves put in the situation of either selling themselves into servitude, starving, or going to jail. Is this slavery?

What about the poor peasants? By the 15th centuries Serfs were becoming rarer, but they still existed into the 19th century as well. A serf was a person who was bonded to the land he worked on for a master for the term of his life. The serf could not leave the land without permissions and effectively worked for free; though, they could often keep some portion of their labor and was entitled to some protections. Is this slavery?

I do not seek to answer if these are slavery or not, though I do hope you have thought about what it means to be enslaved and what slavery is. I only really sought to illustrate a very important concept: Slavery is an incredibly broad term that describes many different social conditions over many different time periods. While it may seem on the surface that there is a long thread of slavery going back to, and likely beyond, the Roman empire, this isn't really the case.

Slavery for African people in the American colonies did not start how we usually envision it. The first African slaves in the American colonies had more in common with Indentured Servants than the Antebellum slave. They usually lived in their own part of town, could legally own property, and were entitled to freedom. But the condition of the non-white slave degrades over time until it more closely resembles what we recognize taking place in cotton fields.

What is clear though, is that chattel slavery of the late 18th and 19th centuries had taken on a new far more sinister characteristic that these earlier forms of forced labor did not have. While wives, children and indentured servants may have been subjected to great depravities at the hands of their husbands, fathers and masters, they were still people in the eyes of the law. An African descended slave in America at this time was considered literal property, sub-human, and had almost no legal rights. They could effectively be murdered at will by their masters, though there were some social protections against this. By the 1850s, most American states had passed laws making it impossible to free slaves, outlawing the teaching of slaves, and removed most any remaining protections slaves may have had under the law.

So why does slavery seem to suddenly go away in the late 19th century? I'd disagree with that question. I don't think you can consider the abolition of slavery to be quick. In both England and the American Colonies/United States people had been pushing for abolition since at least the 1500s. This is about the limit I'd feel comfortable discussing, but I have to imagine the struggle for abolition among the legislature types goes back further. More importantly, this kind of thinking ignores that the Abolition movement begins with the first slave. We cannot only look to the actions of benevolent white abolitionists. When speaking of American history this generally means people in Africa. Enslaved people in Africa, captured by Africans, and sold to Portuguese resisted becoming enslaved. They resisted being put on boats. They resisted on the boats. They resisted when sold in the American colonies, and they resisted until they could resist no more. Without the constant resistance and agitation of enslaved people there would have been no abolition movement. Furthermore, freed slaves fought tirelessly for the abolition of their sometimes literal brothers in bondage.

There were also European descended people fighting for abolition. Quaker settlers in North American and the Caribbean during the 17th century decried all forms of slavery and forced labor wherever they encountered it. Quaker societies were vital to the early struggle for freedom and played an important role in shaping public discourse through the late 18th century. It is also worth noting that indentured servitude has largely died off on its own by the 1760s in the American Colonies. The rising availability of enslaved black people and massive growth in jobs reduced the need for unskilled white labor.

The debates surrounding abolition grew through the 18th and 19th centuries. People like the escaped slave Frederick Douglass toured the United States and Europe in the 1840's and 50's speaking out against slavery. Abolition groups and newspapers grew in popularity in the north, and political movements moved towards restricting slavery, often seen as the first step towards ending it. There are also small legal steps taken to weaken slavery. The British Empire began to close the slave trade in 1807, and in the same year the US Federal government outlaws the importation of slaves. Northern states introduced laws that gradually freed slaves early in the 1800s.

In the 19th century the movement towards abolition grows for many reasons that I will try to briefly summarize. The United States of the early 1800's was a Republic moving towards greater Democratic reforms. In 1790 any naturalized white male could vote. Subsequent and through the 1840's property requirements begin to be relaxed and more and more white men gain the right to vote. With this comes the question of what it means to be a citizen, and what it means to not be a citizen. If a citizen if a free white male then an enslaved black person was a kind of anti-citizen. Some people who became associated with the Free Soil Party in the 1840s believed that slavery should be contained and eventually die off to allow free white men to spread into the west. In their mind slavery not only morally debased white people by making them lazy, but it kept other free white men from gaining economic independence. Free Soilers and others who wanted to restrict slavery clashed with Southern Democrats who wanted to expand slavery over issues like the Annexation of Texas, the War with Mexico, and the Kansas-Nebraska Act from the 1840's through the 1850's.

By the time slavery is formally abolished in 1865, the struggle for freedom has been going on for hundreds of years. Its end may seem sudden, but it was a long process taking place in fields, in houses, in courts, in legislative chambers, on the high seas, and in Africa. Slavery also cannot be viewed as a single condition persisting through all time. Slavery is a term that means many things in many places.

Further Reading

Manisha Sinha - The Slave's Cause

Davis - Inhuman Bondage

Ira Berlin - Generations in Captivity

Your question is global, which would require a global answer. I'm an Americanist, so that's what I'm competent to answer on. /u/nilhaus has already given a good take, but I'm going to come at it from a little different angle. American slavery is something that white people do mostly to black people and on a usually smaller scale to Native Americans. I can't speak to the second category of slavery at all, but the first is my wheelhouse.

It's true that the first abolitionists in this field are black people, both in their capacities resisting enslavement for themselves and in their joint activity to undermine it. They're not powerless under slavery but with the exception of Haiti, they do lack the power to abolish it. That's a privilege that white people reserved to themselves and they were not in any rush.

By the second half of the seventeenth century, slavery is consolidated and increasingly codified by law in Britain's North American colonies. What Winthrop Jordan calls the double perpetuities become the norm: a person is enslaved forever and their children are also enslaved forever. The situation before that is much more complicated and unclear due to ambiguous and scant records, but it looks like the law followed the reality on the ground rather than established it. That said, the transplanted Englishmen in North America (with the important exception of some South Carolinians) did not have experience back home with slavery. They certainly knew of it, but lacked a recent record of practice. They worked it out as they went.

Until the system solidifies, there doesn't seem to be much white resistance to it. As it does, that emerges. The first signpost here is Samuel Sewall's The Selling of Joseph, a pamphlet he published in Massachusetts in 1700. Sewall's objections are not taken up, but through the first half of the eighteenth century there are isolated church protests and debates about the morality of slavery. Religious uneasiness about the institution is probably strongest among Quaker and Methodist denominations, but even within them it's often checked by a more conservative mainstream.

Things shift as the imperial crisis heats up in the middle of the century. The colonists denounce the thing the British are doing to them as reducing them to slavery, which indicates that they understand slavery as something bad and wrong. They had done so from way back, with condemnations of enslavement by the Barbary pirates being a big focus of English thought on the subject. But those were white, Christian Englishmen being enslaved. Here they fear the same. Those objections had not translated over into complaints against enslaving black people. At this time, they begin to do so. Protests for freedom and independence, which partake of a language of notionally universal rights, indict slavery in a a new way and somewhat link up with the previous religious objections. This is by no means a one-for-one translation; Quaker pacifists are increasingly antislavery but also generally not all-in on the Revolution.

Still, the contradiction troubles whites enough that when combined with previous qualms about slavery a kind of momentum develops to do something. This leads to the first American emancipation: Pennsylvania's gradual emancipation act of 1780. (Efforts to terminate slave importation precede this and are aimed at ending slavery, but don't attack it directly in the same way.) It frees no one immediately, but rather frees those born after its passage once they age out...at twenty-eight. They're indentured servants until that point, which no one is going to reach until 1804. There are repeated moves afterwards to enact immediate emancipation and also free the slaves born too soon to fall under the law. PA would show slaves on its census returns up through 1840. The story is broadly similar in most of the North and this trend never reached farther south than Virginia. There it just barely got its foot in the door and only briefly.

What changes in the nineteenth century US isn't entirely intuitive. It seems that most Northern white opinion believed that something was being done or had been done about slavery and it was on its way out. That's true for them all at home, effective with New Jersey's 1804 gradual emancipation law. Slavery is in steep decline and seems to be fixing itself. A degree of complacency sets in, combined with an expectation that the South would follow along.

The South did not. Before 1863, Southerners probably came closest to emancipation in the 1780s and 1790s and that wasn't very close or very many of them. Instead they became more and more invested in slavery and dedicated to continuing and expanding it forever. Religious and political sentiments don't do it for them and instead they work on systematizing a thoroughgoing proslavery ideology. This includes pretending that slavery is benevolent to the slaves as well as defending it as a morally superior social and labor system to paying wages.

These developments don't go unnoticed in the North. By the 1810s, they've noticed that there hasn't been any action on the South's part. Instead the opening up of the inland South to cotton cultivation has given slavery a huge boost in an environment where it hadn't been doing too badly in the first place. To the Northerners watching, this seems like a breech of trust. They agreed, informally, back during the Revoution that they would quiet down complaints against enslaving blacks with an understanding that the white South was going to see to it. Or that's what they thought they agreed to, anyway. That the South hasn't held up its end of the bargain, but keeps getting all the extra power the Constitution granted to it, is not a-ok with them.

This all erupts into the Missouri Crisis in 1819. Missouri wants to be a state and has slavery. A New York congressman, James Tallmadge, sees that MO has about as many slaves proportionately as New York did when it enacted gradual abolition in 1799. Why not help Missouri get rid of slavery and bring a little balance to the Union? The South responds with outrage that threatens to split the nation's only national political party. It's also an issue where the nation's remaining, much-diminished other party, the Federalists, has a bit of an edge over the Republicans (Jefferson's, not Lincoln's) in the parts of the North where they're still popular. The effort to end slavery in Missouri fails and with it, powerfully discredits the idea of working within the system to end slavery.

Antislavery forces are in disarray afterwards and the bisectional political coalitions that form and re-form out of the wreckage largely close them out of meaningful influence. There are still antislavery people in both parties -somewhat more the Whigs as they emerge- but the constructed landscape of US politics makes major gains against slavery virtually impossible. This takes us to about 1830, at which point the movement for immediate, uncompensated emancipation comes to the fore as an antislavery vehicle. This movement is avowedly anti-political, because they saw how working in the system worked out for them.

Between about the Revolution and the emergence of immediateist abolitionism, there are a few major social changes in the North to go with the changing political environment. The first is the rise of political democracy for white men as the accepted norm. This dates back to about the Revolution and puts severe strains on the old, elite-oriented view of politicians as wise elder sort of figures who are singularly endowed with civic virtue. Now those capacities are more vested in the white male populace as a whole, who reject the hierarchical attitudes embodied in calling one's employer "master" and accepting work titles like "servant." Instead "bosses" have "help". As restrictions on suffrage drop or become nominal for whites (while remaining or increasing for black men) slavery comes to look increasingly antithetical. Wage labor becomes a good in this environment, virtuous in its own right and slavery so acquires a new level of infamy in white minds. This is also, it should be said, accompanied by a marked increase in white supremacy among the same set. As they're losing some autonomy by now laboring more for wages and for other men, these same people assert that they are equals because they are white in new ways. To have a master or be a servant becomes a specific capacity of black people, which they want nothing to do with. This is a change that works both ways, proslavery and antislavery but both markedly antiblack.

The second change is a bit harder to date because there are more successive waves of it, but we must also acknowledge the rise of an evangelical form of Christianity. At the time, this is mostly a reforming religiosity that seeks to improve and perfect human mores and society. Antislavery is a big focus, along with opposing Indian Removal, boosting for temperance, and a passel of related reform causes. Antislavery politics were't invented in that reform foment, but they were re-invented. Much of the excitement here happens in the Burned-Over District of upstate New York, which got its name from the successive waves of revivalism and the new denominations and social movements it spawned. Besides the social reform causes already mentioned, the Mormons, the Adventist family of denominations, and various utopian communities all arise from this era.

The abolitionist fruit of that moment soon re-engage, warily, with politics through third parties and keep the flame alive until the silent consensus of the two national parties to protect slavery cracks up over Texas Annexation and the resulting Mexican War. Then mainstream antislavery politics becomes possible again and the movement largely funnels its energies into that channel. This happens first through the Liberty Party, then the Free Soilers and Conscience Whigs, and finally through the Republican party.

I started typing out a response and then became aware that the tone of my post came off as being indifferent to the practice of slavery. So I want to preface my comment with a disclaimer.

Slavery/indentured servitude/serfdom is one of the most abhorrent practices that speaks to the darkest nature of our character. A practice that continues to the modern era and exists in many forms to this day (labour practices in Qatar, human trafficking surrounding the cocaine and sex worker trades, the notorious blood diamond operations in Sudan) slavery and the commodification of people lays at the beating black heart of runaway capitalism.

Yale university's open course on the us civil war and the failure of reconstruction taught by David Blight is an amazing series on the courses, consequences and brutality of institutionalized slavery in the south which I recommend to anyone, along with the memories of Frederick Douglass; one of the most elegant voices on the struggle against slavery in the US.

As for OPs question, advanced applications of the steam engine is probably a pretty good place to start when determining why slavery as a practice began to wane. The industrial revolution ushered in a technological substitute for menial hard labor that was a necessary component of large scale production. And while I don't mean to disparage or underplay how noble or widespread anti-slavery movements and voices had been for hundreds of years; there remains the question of neutralizing the limitless inertia of money that fueled slave trading.

Prior to that, units of energy was a measurement of pairs of hands. Given how prevalent machinery is in our lives today at even a household level (washing machines, lawn mowers, roto-tillers, etc) it's very easy to not realize how many large-scale, energy intensive processes necessarily were hand driven. These hand driven systems of production involved labor so undesirable, punishing, and menial, that no wage system could offer both a fair reward for the labor done and also a final product that could be afforded by anyone. I would argue that until the industrial era of machinery offered an economic alternative, institutions of capital could never allow a meaningful shift away from coerced labor.

Lewis mumfords "technics and civilization" is a survey of the history of the integration of technology into society (beginning with the institutionalised village clock in church towers in town squares) and a great source on the subject of the question.

I'm a dilettante at best but this is one of my favorite topics I studied in undergrad and still one of my favorite subjects for pub talk. I'm not sure I can do it justice up to mod standards but I'll try to relay what I was taught.

My understanding of what triggered the change is based on what I studied at Hamilton College under Professors Ambrose and Paquette, based on their own long careers but primarily influenced by the work of Genovese.

I think this is a difficult question to answer because it's at the crossroads of so many different historic trends. It was not 'sudden', as you imply in your question. It was the result of a perfect storm that brewed over the course of at least a couple hundred years. I think the short answer to your question would be: the advent of capitalism, but I'll try to start closer to the beginning.

Start by looking at two things at once: the predominant economic system and the prevailing religious beliefs in the West long before the tide began to turn against slavery, and then leading up to what became a widespread turn against it.

Let's start by talking about religion. We could go way, way back, but let's start with Calvinism. They broke away from the Catholic Church in the 16th century and their defining beliefs were total depravity and predestination. In other words, us humans are slaves to our sin. We can only be saved through the grace of God, and that's not up to us - it's up to God to decide who has a good life and who has a bad life, who goes to heaven, who goes to hell. It would be folly and hubris to think us humans could control our own fates.

Now let's look at the economic system of the time. We in the West are only just crawling out of the feudalism of the Dark Ages, where we're tied to the fates of our families. If we're serfs we're tied to the land. If you have really bad luck, you die from bubonic plague or, like, a tiny cut on your finger that gets infected (thank god for antibiotics). I mean, average life expectancy in the UK in the 16th century was 35 years old. You better believe the average person did not feel in control of their own fate.

But then - tah-dah! - we have the Industrial Revolution. Things change big time, enough to completely transform world views in those parts of the world that were impacted by the Industrial Revolution and capitalism - the UK, the US Northeast, but notably, NOT the American South. So while you have people like Mary Wollstonecraft and Orestes Brownson fueling religious and philosophical movements like Transcendentalism, the labour movement, feminism, the religious fervor in the Burned Over District (a place also remarkably transformed by the Industrial Revolution), in the antebellum South, where slavery as an economic system was still perfectly profitable - if not more so than ever - (white) people remained staunchly Methodist, clinging to tenets of Calvinism that insist that our roles in life were assigned and handed to us by an all-powerful God. How dare we argue against a divine will that clearly chose for me to be a slaveholder and you to be a slave? We tend to believe what we want to believe, yes?

But capitalism and all its attendant philosophies and religious ideas could not be stopped. Now that a man (or woman) could choose to work a job in a factory and earn a living for himself, now that a person's worth could literally be measured by how much work they were able to put on the clock for a day, slavery just didn't make much sense. But what did make a lot of sense to those whose lives were changed by capitalism are ideas like: choosing salvation. It’s not God’s decision! It’s an act of personal will - it's my choice to ask for salvation and work to be a good person. (How capitalist is that?) Even the word 'individual' didn't come into popular use until the 19th century when many post-Industrial Revolution thinkers began to find the word useful for the ideas they now wanted to articulate.

This could be way more in-depth, but it’s only my attempt to summarize an entire course I took with Ambrose and Paquette on the antebellum South. I do encourage you to read more of Genovese if you really want to go down this fascinating rabbit hole.

That is, as others have stated, an incredibly complex question, but one that brings many fruits. That is the question I bring to my high school students in order to introduce the Idea of Historiography and, indeed, if you look around the thread, you will see many different ideas stemmed from the many different traditions within the study of slavery and abolition. Just keep in mind that I am going to reference mainly the country that led the abolitionist charge from the 18th century, Great Britain.

I want to bring here, in a shorter format, Eric Williams' answer to this question in Capitalism and Slavery, more because of its ramifications in the politics of history rather than the argument itself.

I am going to start off by saying that Williams' argument was pretty handily refuted by Seymour Drescher years later, but it bears explanation anyway: At its core, Williams believed that the industrial revolution that was happening in England at the turn of the 18th century quickiy made the plantation system in the Caribbean economically obsolete. The very same social systems that led the Industrial revolution and allowed the development of british capitalism (think Quakers, the capitalists themselves, etc) now turned against the less efficient but at the time still politically prominent colonial groups within the British Empire, culminating in the abolition of british colonal slavery in 1838 and the firm ushering of these groups as the political leaders of the BE.

Williams' argument and logical conclusion is that abolition was economically motivated, and as such it faces its limits when the economical situation was again in Britain's favor, such as in American slave-grown cotton and Brazil's slave grown sugar and coffee. This all was put in check by Seymour Drescher's very thorough economic analysis in Econocide, when it was found out that colonial slavery was indeed profitable up until 1838 when it was abolished. Rather, Drescher's refuttal of Wiliams has the rather chilling implication that slavery and capitalism can perfectly coexist If you wantto read more on that, check Dale Tomich's book Through the prism of slavery

What's really interesting is that despite Williams' academic refuttal, he still is immensely popular (if you are not american, and especially if you are latin american, the narrative you learn in school about the end of chattel slavery comes from him). In a way, it is easy to see why he (or his narrative) is still taken up by sectors of politics and academia alike: Williams provides a link between a seminal experience in many countries in the third world (slavery and abolition) with another one that followed suit (industrial imperialism). While I cannot say that Williams has directly influenced him, Ha Joon Chang's work Kicking away the ladder strikes many of the same notes Williams has - some Countries move faster than others on their economic development for some reason or another, and when they are making a transition to the next stage, they kick the proverbial ladder away from the less developed countries (eg. Britain "fueled" the industrial revolution with capital acquired by the slave trade and slave colonies during the 18th century, but as they industrialized, they denied that possibility to countries trying to take the same western development path by an ostensibly (but fake) moral reason).

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