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Diocletian (born Diocles) Gaius Aurelius Valerius Diocletian (245-313) was Roman Emperor from November 20, 284 to May 1, 305.

This bronze coin, showing his profile, was minted during Diocletian's rule.

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.

Dioceses of Diocletian
Name Territories
OriensLibya, Egypt, Plaestine, Syria, and Cilicia
PontusCappadocia, Armenia Minor, Galatia, Bithynia
Asia (Asiana)Asia, Phrygia, Pisidia, Lycia, Liydia, Caria
Thrace MoesiaeMoesia Inferior, Thrace
MoesiaeMoesia Superior, Dacia, Epirus, Macedonia, Thessaly, Achaea, Dardania
AfricaTripolitana, Africa Proconsularis, Numidia, part of Mauretania
HispanieMauretania Tingitana, Baetica, Lusitania, Tarraconensis
ViennensisNarbonensis, Aquitania, Viennensis, Alpes Maritime
GalliaeLugdunensis, Germania Superior, Germainia Inferior, Belgica
BritanniaeBritannia, Caesariensis
ItaliaLiguria, Venetia, Alpes Cottiae, Alpes Graiae, Raetia
PannoniaePannonia Inferior, Pannonia Superior, Noricum, Dalmatia
SuburbicariaUmbria, Campania, Sicilia, Corsica, Sardina

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Preceded by:
Carinus (283 - 284)
Roman emperors
Followed by:
Constantius Chlorus (305 - 306),
Galerius (305 - 311)