In AD 200, the Roman Empire seemed unassailable, its vast territory accounting for most of the known world. By the end of the fifth century, Roman rule had vanished in western Europe and much of northern Africa, and only a shrunken Eastern Empire remained.This was a period of remarkable personalities, from the philosopher-emperor Marcus Aurelius to emperors like Diocletian, who portrayed themselves as tough, even brutal, soldiers. It was a time of revolutionary ideas, especially in religion, as Christianity went from persecuted sect to the religion of state and emperors. Ultimately, this is the story of how an empire without a serious rival rotted from within, its rulers and institutions putting short-term ambition and personal survival over the greater good of the state.