The story of slavery's abolition in Great Britain and the United States is well known. But that chapter in the history of oppression was made possible by the slavery that had been practiced in Africa for centuries. So, who abolished slavery in 19th-century Africa? When historian Lamin Sanneh set out to explore this question, he came upon some surprising answers.

Sanneh's latest book, Abolitionists Abroad: American Blacks and the Making of Modern West Africa, tells a hitherto almost unknown story of the end of slavery: what black Christians did in Africa. Born to a Muslim family in Gambia, West Africa, Sanneh became a Christian while in high school. He earned his Ph.D. in Islamic studies at the University of London, and is the D. Willis James Professor of Missions and World Christianity at Yale University. CT senior writer Tim Stafford reached Sanneh at his home in Connecticut.

Your book tells how a small band of freed slaves from America planted a Christian society in Freetown, Sierra Leone, that had revolutionary effects. Who were these ex-slaves?

For the most part, they were American blacks who, as a result of the American Revolution, were eventually repatriated through Nova Scotia to West Africa. There they planted successful colonies for freed slaves, becoming antislavery champions. In the medieval period, European missionaries went to Africa with the aim of converting the aristocracy, on the ground that if you convert the chiefs, that would inspire the rest of society to become Christianized. This "top-down" approach was tried in Africa for 300 years (between 1475 and 1785). It didn't have much of an impact.These new antislavery blacks, though, started from the bottom up, first converting former slaves, then instructing ...