Africa has been coveted for its riches ever since the era of the Pharaohs. In past centuries, it was the lure of gold, ivory, and slaves that drew fortune-seekers, merchant-adventurers, and conquerors from afar. In modern times, the focus of attention is on oil, diamonds, and other valuable minerals. Land was another prize. The Romans relied on their colonies in northern Africa for vital grain shipments to feed the population of Rome. Arab invaders...