Actually, the first governmental units to reject slavery were the legislatures of the states of Pennsylvania and Massachusetts in 1780 when they passed legislation to end slavery in their states. Granted, they were "gradual emancipation" measures, but they were a very radical step at that time in keeping with the ideas of liberty that the war was being fought over. This was at the height of the Revolutionary War when the British were still the driving force in the Atlantic slave trade and before the British conscience kicked in.
The first nation state were the Austrian Habsburgs 1781...modern slavery that is.
It had ebbed and flowed all over the planet for as long as recorded history. It was estimated that during the Roman (Holy or otherwise) Empire that half were in servitude....Europe in the middle ages too.
Slavery then more often died because of other reasons than emancipation.
More fun facts:
27 million slaves estimated today. In today's dollars...a good young adult african slave in New Orleans in 1820 cost 40,000 dollars. The same slave today in the Sudan is about 90 dollars. That is amazing.
Actually, the the first governmental unit to reject slavery was .... Georgia! In January 1735, the state Trustees passed "An Act for ren'ring the Colony of Georgia more Defensible by prohibiting the Importation and Use of Black Slaves or Negros into the same", which was approved by the Privy Council 3 Apr 1735. The act prohibited the importation and use of slaves after 24 Jun 1735, refusing to permit 'such a horrid crime.' James Oglethorpe wrote in 1734, '[s]lavery, the misfortune, if not the dishonor of other plantations, is absolutely proscribed [forbidden]. Let avarice defend it as it will, there is an honest reluctance in humanity against buying and selling, and regarding those of our own species as our wealth and possessions.'