An Illyrian of low birth, Diocles rose through the ranks to the consulship. He was chosen by the Army in 284 to replace Numerian and after the assassination of Carinus became sole ruler of the Roman Empire. He changed his name to Diocletian upon his ascension.
Diocletian felt that the system of Roman imperial government was unsustainable in the face of internal pressures and a military threat on two fronts. He gave Maximian the title of Caesar, which was the traditional form in which an emperor (Augustus) designated a successor. However, Diocletian soon made Maximian an Augustus as well. The imperial power was now divided between two people. Diocletian's sphere of influence was the east, and Maximian's the west.
The two men established separate capitals, neither of which was at Rome. The ancient capital was too far removed from the places where the empire's fate was decided by force of arms. While improving the ability of the two emperors to rule the empire, the division of power further marginalized the Senate, which remained in Rome.
In 292, Diocletian and Maximian each appointed a Caesar (Galerius and Constantius, respectively). However, these were not merely successors - each was given authority over roughly a quarter of the empire. This form of government is known to historians as the Tetrarchy.
Considering that during the half-century preceding Diocletian's ascension the empire had been in a constant state of simmering civil war, with (according to one scholar) a new emperor every two and a half years on average, it is remarkable that the Tetrarchy did not immediately fall apart due to the greed of any one of the four emperors. The opportunistic nature of Roman imperial politics did eventually cause the disintegration of the Tetrarchy and the reinstitution of one-man rule, but this was not until the 320s.
In 305, Diocletian retired to his palace at the administrative center of Salona on the Adriatic Sea, near the modern Split, Croatia. He was the only Roman emperor to remove himself from office; all of the others either died of natural causes or were removed by force.
The last and greatest persecution of Christians by the Roman Empire was carried out during his reign, though Galerius carried it out more avidly than Diocletian himself.
|Oriens||Libya, Egypt, Plaestine, Syria, and Cilicia|
|Pontus||Cappadocia, Armenia Minor, Galatia, Bithynia|
|Asia (Asiana)||Asia, Phrygia, Pisidia, Lycia, Liydia, Caria|
|Thrace Moesiae||Moesia Inferior, Thrace|
|Moesiae||Moesia Superior, Dacia, Epirus, Macedonia, Thessaly, Achaea, Dardania|
|Africa||Tripolitana, Africa Proconsularis, Numidia, part of Mauretania|
|Hispanie||Mauretania Tingitana, Baetica, Lusitania, Tarraconensis|
|Viennensis||Narbonensis, Aquitania, Viennensis, Alpes Maritime|
|Galliae||Lugdunensis, Germania Superior, Germainia Inferior, Belgica|
|Italia||Liguria, Venetia, Alpes Cottiae, Alpes Graiae, Raetia|
|Pannoniae||Pannonia Inferior, Pannonia Superior, Noricum, Dalmatia|
|Suburbicaria||Umbria, Campania, Sicilia, Corsica, Sardina|