Let us return to where this book began: on a hill, three miles outside of Nicomedia, where Diocletian addressed an assembly of officers and soldiers and announced that he was stepping down from his position as Augustus. The abdication of the Augusti in a period of stability was unprecedented in Roman history. Tetricus had abdicated in response to Aurelian’s invasion of Gaul, and Vitellius and Didius Julianus had similarly offered to abdicate in their attempts to avert their demise in civil war. However, Diocletian and Maximian were not faced with internal strife when they left office. Britain had returned to their control in 295 or 296, Egypt in 298, and by 299 Diocletian and Galerius had forced the empire’s greatest enemy, the Persians, to agree to a humiliating peace treaty. The Augusti may well have abdicated to fulfil the expectation that the Caesars would in turn become Augusti, and it is plausible that Diocletian also wished to supervise his own succession, controversial as it was. As an interventionist ruler, to directly preside over his succession was in character. This was, after all, the emperor who expanded the armies, restructured and augmented imperial and provincial administration, ordered the first codification of rescripts, reformed and revalued currency, introduced systems of census and indiction, introduced new forms of imperial imagery and ceremonial, standardised numismatic iconography and issued edicts on prices and religion. With all that being said, in making the decision to abdicate, perhaps Diocletian’s background was on display. For Diocletian, was the role of imperator the final appointment in a military career hinged on promotion and ending in retirement? Was Diocletian, as Umberto Roberto suggests, essentially a soldier retiring to his land to plant cabbages after a lengthy service or militia?

The thought is an enticing one, and it returns us to a theme of this book: the military professional as emperor. As military professionals who had become emperors, Diocletian and his colleagues were the successors of Maximinus I, Postumus, Claudius Gothicus, Aurelian, Probus and Carus. These emperors were men who had climbed the ranks of the armies, some of them from relatively humble backgrounds, to become powerful members of a ruling circle replete with equestrian military officers. This demographic change in the imperial leadership would have affected imperial politics and the representation of emperors in contemporary media.