Diocletian (242/245 – 311/312) was Roman emperor from 284 until his abdication in 305.
Diocles, later known as Diocletian, was born of humble origins in the Balkan province of Dalmatia. His official birthday was 22 December, and his year of birth has been estimated at between 242 and 245 based on a statement that he was aged 68 at death.
Following in the footsteps of his predecessors, Diocletian rapidly ascended the ranks of the Roman army and joined an elite unit in the Illyrian army. In 283, he accompanied the Roman emperor Carus to Persia, serving as part of the imperial bodyguard or protectores domesticis. He continued in this role under Carus' successor and son Numerian, who died a year later. Although there were suspicions about Diocletian's involvement in Numerian's death, the blame fell on the Praetorian Guard commander Arrius Aper, Numerian's father-in-law. Diocletian, seeking vengeance for the emperor's death, killed Aper in front of his own troops.
After having been proclaimed emperor in November 284, Diocletian crossed the Bosporus Strait into Europe. There, he confronted and defeated Numerian’s brother Carinus, who served as co-emperor. This triumph marked Diocletian's complete control of the empire, and he assumed the name Gaius Aurelius Valerius Diocletian.
As governing the whole Roman Empire was challenging due to its immense size, Diocletian strategically divided it into two parts, appointing his son-in-law Maximian as Caesar in the West. One year later, Maximian was promoted to Augustus, assuming the name Marcus Aurelius Valerius. Diocletian retained the position of emperor in the East and maintained a senior position, granting him the authority to veto any of Maximian's decisions.
However, peace in the empire did not last long. Diocletian spent five years campaigning in the eastern half of the empire, achieving victory in 286 and earning the title of Germanicus Maximus. In the West, Maximian contended with the North Sea fleet’s commander Carausius, who declared himself emperor but eventually met his end at the hands of someone under his own command.
To address the issue of succession, Diocletian implemented the tetrarchy, splitting the empire with two Augusti (Diocletian in the East and Maximian in the West) and a Caesar designated to succeed each Augustus. Each leader administered their territory with its capital, while all Caesars were accountable to Augusti. Maximian appointed his praetorian commander Constantius as his Caesar, whereas Diocletian chose Galerius.
With stability restored, Diocletian turned his attention to domestic affairs. To minimize revolt risks in outlying provinces, he doubled the number of provinces to one hundred and organized them into twelve dioceses, each governed by vicars without military responsibilities. As more funds were needed to support provincial restructuring and expanding the military, Diocletian revised the tax system. He ordered a new census, aiming to determine the population, land ownership, and agricultural productivity across the empire. Moreover, to curb runaway inflation, he issued the Edict of Maximum Prices, which aimed to set fixed prices for goods, services, and wages.
While contending with financial and border security challenges, Diocletian dealt with the rising influence of Christianity, which he perceived as an obstacle to stability. In 297, he ordered all soldiers and administrative personnel to sacrifice to the gods; those who resisted were compelled to resign. A few years later, in 303, he issued a decree for the destruction of churches and Christian texts. During this period known as the Great Persecution, the repercussions were severe for Christians, with prominent clergy members being arrested and given the ultimatum to either sacrifice to the pagan deities or face death. The prolonged persecution eventually came to an end in 305.
Abdication & Death
In 303, Diocletian fell seriously ill, forcing him to abdicate in 305. He retired to his palace fortress in Spalatum, present-day Split in Croatia. Diocletian also convinced Maximian to step down, resulting in a joint abdication. This paved the way for Constantius and Galerius to ascend as the new Augusti, while Maximinus and Severus were appointed as the new Caesars. Although briefly emerging from retirement in 308, the emperor spent his remaining days in his palace, passing away in October 311.
Diocletian’s tetrarchy eventually disappeared. After years of conflict among successors, Constantius' son, Constantine I, achieved the reunification of the empire following the Battle of Milvian Bridge in 312. Establishing his rule in a city that would bear his name, Constantinople, Constantine granted Christianity due recognition and even embraced the faith. In 476, with the fall of the Western Roman Empire, the East, while retaining echoes of the Old Rome, experienced a revival as the Byzantine Empire.