As a member of Mauritania's slave-owning class, Abdel Nasser Ould Ethmane could have had anything he wanted as a present for his circumcision ceremony: a toy, money, a camel, or, as his brother would choose, a bicycle. But the 7-year-old wanted something more sinister. He chose Yebawa Ould Keihel, a young boy with skin the color of coal. At that moment, Abdel became a slave master. It's an experience that's common here in Mauritania, a vast country in West Africa's Sahara Desert where activists and the United Nations estimate 10% to 20% of people are enslaved -- usually dark-skinned people...