Slavery operated in the very first civilizations on our planet eg Sumer in Mesopotamia, 5,500 years ago!!! Read Hammurabai’s Code about 1860BC, where slavery was clearly a well-established cultural institution.
Slavery came about with the development of agriculture about 10,000 BC, since it required a fit, and ultimately, profitable labour force. Many of the early slaves were captives of war.
Native African rulers, or warlords, collaborated with white European and Arab slave traders from the seventh century AD onwards.
The Portuguese and Dutch became involved from the 1400’s.
British merchants became involved in the 1700’s before it was outlawed and the Royal Navy actively worked to end the trade a hundred years later.