All histories of Rome are histories of empire. Her rise to power, the long peace, and the even longer decline together form the background to every story told about the Romans. My subject, however, is empire itself. How did it grow? What enabled it to resist defeats and capitalize on victories? Why did Rome succeed when its rivals failed? How did empire survive crises, dig itself in, and replace chaotic campaigns of conquest with stability? How did empire come to coordinate the great flows of wealth and populations on which it depended? How did it evolve to face new needs and new threats? Why did it falter, regain its balance, and then shrink under a series of military blows until it was, once again, a city-state? What circumstances and technologies made the creation and maintenance of an empire possible, in just this place and just at that time? What institutions, habits, and beliefs suited Rome for the role? And what did the fact of empire do to all the beliefs, habits, and institutions with which the world had been conquered? What part did chance play in its successes and its failures?
The long arc that stretches from a scatter of villages on the Tiber River to a medieval city on the Bosporus Straits dreaming of ancient glory takes a millennium and a half. Telling that story in a single volume is perhaps a crazy endeavour, but it has also been an exhilarating one. Perhaps Roman history has no special claim on us, among the many periods of the past we can think about, and that have shaped our world. But as a student I felt the fascination of studying something so vast, an entity that stretched over so much time and space. What could sustain a human enterprise conceived on such a vast scale? How could anything human last so long? Our own world experiences change at an extraordinary rate. Earlier generations, confident of the permanence of their own empires and of the uninterrupted march of progress, were spellbound by Rome’s decline and fall. For us it is the longevity of Rome that grasps the imagination. My own fascination has not diminished since my student days. Even now the Roman world still sometimes feels like a vast sandpit in which I can play, or else a huge historical