When Englishman Thomas Pellow was 27, he led a slave-hunting expedition to the West African coast. His orders were to plunder the villages, kill the adults and capture the children. But Pellow was not a mercenary employed in the transatlantic slave trade, which sent millions of its victims across the ocean. He was a slave himself – taken prisoner as a child by the Moroccan Sultan Moulay Ismail. And 300 years ago, he was far from alone. The sultan owned an estimated 25,000 European slaves, many seized in raiding expeditions on the south coast of England as well as countries...