Slavery is as old as human history. The first tablets of writing made six thousand years ago in Sumer (modern-day Iraq ) mention slavery—the practice of one person forcing another to work for no compensation. And slavery has appeared almost universally throughout history, from the slaves who built the great pyramids in ancient Egypt to the slaves who worked the plantations in the southern United States in the nineteenth century.

In Africa, slavery had been a common practice among tribes for centuries. Slaves were usually taken in war raids and were used to enhance the power of tribal chiefs. When Europeans first arrived on the African coast in the fifteenth century, chiefs were willing to bargain their human wealth for gunpowder, weapons, liquor, and cloth. The slave traders took only the youngest and strongest men and women because they...