Of very humble Dalmatian origin, Diocletian fought as an officer in the emperor Numerian’s army when the latter was killed in 284, at the end of an unfortunate Persian campaign. Diocletian publicly accused the praetorian prefect Aprus of the crime and had him executed, and the soldiers acclaimed him emperor at Chalcedon. The other emperor, Carinus, Numerian’s brother, moved against him and defeated him in Pannonia, but was killed by his own soldiers after the battle. Diocletian, remaining sole lord, and appointed his companion-in-arms Maximian as coemperor in the West, giving him the title of Caesar, in 286 conferring upon him the dignity of Augustus. The organization Diocletian gave to the state tetrarchy provided for the subdivision of the empire between four sovereigns two with the higher rank of Augustus and two with the inferior rank of Caesar, and the introduction of an elective, rather than hereditary, principle of succession.
This complex system, which maintained a hierarchy not only between the Augusti and the Caesars, but also between the two Augusti with Diocletian at the summit of power, was reinforced by a religious aura and organized on the model of the Eastern monarchies: all that pertained to the person of the emperor dominus noster and all court institutions had a sacred character. In 287 Diocletian took the title of Iovius, descendent of Jupiter, giving his colleague that of Herculius i.e., of the stock of Hercules. The two Caesars, Galerius and Constantius Chlorus, appointed in 293, were each invested with the name of their own Augustus, as if to constitute two dynastic lines of divine descent. None of the sovereigns chose Rome as a capital: Diocletian’s seat was Nicomedia, Maximian’s Milan.
Diocletian’s reforms involved nearly every area of state organization: administration, finance, the economy, the army. His notorious persecution was the last systematic sanguinary action against the Christians in Roman imperial history. It took place in the last years of his reign, beginning in 303: the four edicts condemning Christian worship, which were preceded by an anti-Manichean persecution, should be seen as part of an effort to restore the unity of the state, whose religious cohesion was threatened by the incessant progress of Christianity. Diocletian abdicated in 305, forcing his colleague Maximian to do the same. He spent the last years of life at Salona near today’s Spalato, Split, where the remains of his palace still exist; he returned briefly to public life in 308 in an attempt to restore the tetrarchical order, already in crisis shortly after his abdication.