One of the great maxims of history is that it is written by the victors, and nowhere does this find greater support than in the later Roman Empire. Between 284 and 395 AD, no fewer than 37 men claimed imperial power, though today we recognize barely half of these men as 'legitimate' rulers and more than two thirds died at their subjects' hands. Once established in power, a new ruler needed to publicly legitimate himself and to discredit his predecessor:...