What is unique is that it is only in the modern Age of Enlightenment era with its simultaneous beliefs in universal human rights, and rule of law, and property rights, that we have seen the virtual elimination of slavery as an institution. The idea that Age of Enlightenment, classical liberalism required the elimination of slavery is well documented and very traceable to extensive debates and specific legislation, starting in the US, Britain, and France and spreading from there. It is a fascinating history.
I think there were essentially four pulses of freedom that marked the elimination of global slavery. The first three came out of the three heartlands of Age of Enlightenment thinking and institutions - Great Britain, France, and the United States from 1794 to 1865. This was followed by Russia in 1861-66. There was a lot of mopping up to be done here and there over those decades.
But the biggest assault on slavery began in Britain.
The marriage of Christian beliefs and Age of Enlightenment universalism gave the effort its earliest and most powerful boost in Britain. One of the key challenges though, was the reconciliation of simultaneous beliefs marking a cultural transition. The belief in universal human rights (which precludes slavery) had to work through the other Age of Enlightenment precepts - rule of law and property rights. The nut of the problem was how to convert slaves (a then form of property) into free humans while adhering to the law and observing property rights.
Of course the whole process, new in global affairs, was messy.
In the United States, despite the Age of Enlightenment ethos of the Revolution and the inspiration of Thomas Paine's Common Sense, there was a false start. The Founding Fathers were unable to bridge the gap between universal human rights, property rights and rule of law. Without being able to reach agreement in an already fragile set of circumstances, having no credit, and being unwilling to fight yet another war, they incorporated slavery as it then existed into the Constitution, a problem to be resolved later. Jefferson made a small downpayment against this blight by supporting legislation which ended up outlawing the importation of slaves to the US in 1808.
France was the first to abolish slavery by legal edict in 1794 with the promise that slave owners would be compensated for their property. The mechanics of making that happen fell victim to the turmoil of the French Revolution. Seeking to avoid too many wars on too many fronts, Napoleon promulgated a new law in 1802, reestablishing slavery in French colonies which continued until 1848. This was the second false start.
Britain was the most effective in its abolitionist efforts starting circa 1750s and gaining momentum in the 1770s. One of the key sticking points was the cost of abolition. If slave-owners were to be compensated for their usurped property (as rule of law and property rights demanded), how much and who was to pay. This was no small issue. The great majority of the British population still lived perilously close to poverty and starvation in the late eighteenth century. The capital value of the total population of slaves was immense.
Serendipitously, national credit markets also came into existence with the Age of Enlightenment, first in the Netherlands and then across northern Europe. Britain was only able to sustain its freedoms and fighting capabilities during the Napoleonic wars through massive national debt accumulation. This was new, unprecedented and contentious. Despite moral and pragmatic objections to debt, however, national debt did work to accomplish things otherwise impossible to achieve.
The Slave Trade Act of parliament in 1807 laid the first foundation for the eventual global elimination of slavery by outlawing the trade in slaves in its territories. With both Britain and the United State outlawing the traffic in slaves, and with the might of the British Navy behind it, and given the extent of Britains colonial holdings, this, to some degree, eliminated the trade of slaves globally but without addressing the core issue of slave ownership. That had to wait until 1833.
From The Power of Ideas and the Great Emancipation of 1834.
In the Slavery Abolition Act of 1833, Parliament committed the huge sum of 20 million pounds sterling to compensate slave owners for the loss of their “assets.” That was equivalent to 40 percent of the entire national budget (and five percent of Britain’s GDP at the time), requiring the government to borrow most of the 20 million from private sources. It finally paid the loan off just three years ago, in 2015. The former slaves themselves received no payment for their suffering—a lamentable aspect of the law that freed them.Britain squared the circle of liberty, property and rule of law by undertaking the modern equivalent of a $1.6 trillion debt for 182 years to be borne by the whole nation. Remarkable.
France eventually abolished slavery in its colonies (with compensation) in 1848.
The United States ultimately paid for the abolition of slavery in blood. 620,000 dead and cultural scars for another two or three generations. $1.6 trillion was a steep price the British paid but a bargain in comparison.
Russia abolished serfdom in 1861-66, freeing some twenty-five million serfs. The cost of freedom was borne by the serfs themselves, the government imposing taxes on the newly freed serfs in order to compensate their former owners.