Whilst on holiday I read a couple of books about slavery - as one does.
The first was White Gold by Giles Milton, in which he describes the experience of an Englishman, Thomas Pellow, abducted by North African pirates in 1715 and sold as a slave in Morocco, and also the larger story of how around a million Europeans and Americans were captured and sold into slavery in North Africa in the 250 years or so up to the start of the 19th Century (when the British sailed into Algiers, issued an ultimatum demanding an end to slavery and then blew the place to bits).
The second was Rough Crossings by Simon Schama, which is specifically about the American Revolution and the offer made by the British that any slave that made it to British lines and fought for the British would get their freedom - and what happened to them after that. One of the fascinating bits is about how, before the American Revolution began, there was already an anti-slavery movement in England and a number of court cases had effectively ruled that ownership of slaves could not be enforced in England because slavery was not recognised in Common Law, this was widely believed to mean that slavery was illegal.
As you'd expect the Blacks who fought for the British was despised by the slave-owning American 'patriots', quite a few of them were owned by signatories of the American Declaration of Independence ('all men are created equal') and by George Washington. After the war, they were relocated to Nova Scotia and New Brunswick in Canada, where they were fairly abominably treated - especially by the slave-owning Empire loyalists (Americans who were against the rebellion and chose to go to Canada). In the end, many of them were relocated voluntarily to Sierra Leone, where they were repeatedly let down and badly treated but did eventually find a measure of freedom.
The book ends with the anti-slavery movement growing in strength on both sides of the Atlantic until the British banned the trade, then (some years later) freed the slaves (compensation went to their former owners rather than the slaves themselves) and then actively closed down the slave trade of other countries - which is where it ties in to the end of the other book. Sadly, slaves in the southern states of America had to wait another generation before they were freed.
Which all goes to show what a perculiar muddle history is; progressive concepts that we take for granted now, such as democracy, the rule of law, personal liberty, freedom of speech and other basic human rights, were once just tentative ideas competing against other, completely regressive, ideas - and in some countries they still are.
Is freedom something that would just have happened of necessity as human society got larger and more complex (particulary the devlopment of capitalism) - or is it a specific legacy of a combination of historical events and traditions (Athenian democracy, Roman law, Christianity, English Common Law, Magna Carta, the English Civil War, the American Revolution, the French Revolution...) Personally I think it's the former, freedom develops when people are sufficiently well-off to have choices and that's the only way to successfully run highly developed countries.