Diocletian was on to something. After more than two decades as emperor of Rome, during which time he'd put down all manner of threats to bring the Roman Empire back from the brink of collapse and create a lasting peace, he decided, in May 305, that he'd had enough of waging war and managing political squabbles. Diocletian became the first Roman emperor to abdicate the position voluntarily. He'd prepared for the day. He'd built a 50,000-square-foot palace for himself on...