Diocletian
Maximian made Caesar

The assassinations of Aurelian and Probus demonstrated that sole rulership was dangerous to the stability of the empire. Conflict boiled in every province, from Gaul to Syria, Egypt to the lower Danube. It was too much for one person to control, and Diocletian needed a lieutenant. According to Eutropius, Diocletian raised his fellow-officer Maximian to the office of Caesar, making him his heir and effective co-ruler.

The concept of dual rulership was not new to the Roman Empire. Augustus, the first emperor, had nominally shared power with his colleagues, and a formal office of co-emperor (co- Augustus) had existed from Marcus Aurelius onward. Most recently, Emperor Carus and his sons had ruled together, albeit unsuccessfully. Diocletian was in a less comfortable position than most of his predecessors, as he had a daughter, Valeria, but no sons. His co-ruler had to be from outside his family, raising the question of trust. Some historians state that Diocletian adopted Maximian as his filius Augusti, his "Augustan son", upon his appointment to the throne, following the precedent of some previous Emperors. This argument has not been universally accepted. Diocletian and Maximian added each other's nomina (their family name, "Valerius" and "Aurelius", respectively) to their own, thus creating an artificial family link and becoming part of the "Aurelius Valerius" family.

The relationship between Diocletian and Maximian was quickly couched in religious terms. Around 287 Diocletian assumed the title Iovius (Jovius), and Maximian assumed the title Herculius (Hercules). The titles were probably meant to convey certain characteristics of their associated leaders. Diocletian, in Jovian style, would take on the dominating roles of planning and commanding; Maximian, in Herculian mode, would act as Jupiter's heroic subordinate. For all their religious connotations, the emperors were not "gods" in the tradition of the Imperial cult – although they may have been hailed as such in Imperial panegyrics. Instead, they were seen as the gods' representatives, effecting their will on earth. The shift from military acclamation to divine sanctification took the power to appoint emperors away from the army. Religious legitimization elevated Diocletian and Maximian above potential rivals in a way military power and dynastic claims could not.
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