Reign: 20 November 284 – 1 May 305 (20 years, 5 months and 11 days) Whole; then East
Tetrachy

Manner of Death: First emperor to voluntarily abdicate. Died in unclear circumstances, possibly suicide

Diocletian (Gaius Aurelius Valerius Diocletianus, 242/245 – 311/312), nicknamed Jovius, was Roman emperor from 284 until his abdication in 305. He was born Diocles to a family of low status in the Roman province of Dalmatia. Diocles rose through the ranks of the military early in his career, eventually becoming a cavalry commander for the army of Emperor Carus. After the deaths of Carus and his son Numerian on a campaign in Persia, Diocles was proclaimed emperor by the troops, taking the name Diocletianus. The title was also claimed by Carus's surviving son, Carinus, but Diocletian defeated him in the Battle of the Margus.

Diocletian's reign stabilized the empire and ended the Crisis of the Third Century. He appointed fellow officer Maximian as Augustus, co-emperor, in 286. Diocletian reigned in the Eastern Empire, and Maximian reigned in the Western Empire. Diocletian delegated further on 1 March 293, appointing Galerius and Constantius as junior colleagues (each with the title Caesar), under himself and Maximian respectively. Under the Tetrarchy, or "rule of four", each tetrarch would rule over a quarter-division of the empire. Diocletian secured the empire's borders and purged it of all threats to his power. He defeated the Sarmatians and Carpi during several campaigns between 285 and 299, the Alamanni in 288, and usurpers in Egypt between 297 and 298. Galerius, aided by Diocletian, campaigned successfully against Persia, the empire's traditional enemy, and in 299, he sacked their capital, Ctesiphon. Diocletian led the subsequent negotiations and achieved a lasting and favorable peace.

Diocletian separated and enlarged the empire's civil and military services and reorganized the empire's provincial divisions, establishing the largest and most bureaucratic government in the history of the empire. He established new administrative centers in Nicomedia, Mediolanum, Sirmium, and Trevorum, closer to the empire's frontiers than the traditional capital at Rome. Building on third-century trends towards absolutism, he styled himself an autocrat, elevating himself above the empire's masses with imposing forms of court ceremonies and architecture. Bureaucratic and military growth, constant campaigning, and construction projects increased the state's expenditures and necessitated a comprehensive tax reform. From at least 297 on, imperial taxation was standardized, made more equitable, and levied at generally higher rates.

Not all of Diocletian's plans were successful: the Edict on Maximum Prices, his attempt to curb inflation via price controls, was counterproductive and quickly ignored. Although effective while he ruled, Diocletian's tetrarchic system collapsed after his abdication under the competing dynastic claims of Maxentius and Constantine, sons of Maximian and Constantius respectively. The Diocletianic Persecution (303–312), the empire's last, largest, and bloodiest official persecution of Christianity, failed to eliminate Christianity in the empire. After 324, Christianity became the empire's preferred religion under Constantine. Despite these failures and challenges, Diocletian's reforms fundamentally changed the structure of the Roman imperial government and helped stabilize the empire economically and militarily, enabling the empire to remain essentially intact for another 150 years despite being near the brink of collapse in Diocletian's youth. Weakened by illness, Diocletian left the imperial office on 1 May 305, becoming the first Roman emperor to abdicate the position voluntarily. He lived out his retirement in his palace on the Dalmatian coast, tending to his vegetable gardens. His palace eventually became the core of the modern-day city of Split in Croatia.

Diocletian returned to Antioch in the autumn of 302. He ordered that the deacon Romanus of Caesarea have his tongue removed for defying the order of the courts and interrupting official sacrifices. Romanus was then sent to prison, where he was executed on 17 November 303. Diocletian left the city for Nicomedia in the winter, accompanied by Galerius. According to Lactantius, Diocletian and Galerius argued over imperial policy towards Christians while wintering at Nicomedia in 302. Diocletian believed that forbidding Christians from the bureaucracy and military would be sufficient to appease the gods, but Galerius pushed for extermination. The two men sought the advice of the oracle of Apollo at Didyma. The oracle responded that the impious on Earth hindered Apollo's ability to provide advice. Rhetorically Eusebius records the Oracle as saying "The just on Earth..." These impious, Diocletian was informed by members of the court, could only refer to the Christians of the empire. At the behest of his court, Diocletian acceded to demands for universal persecution.
On 23 February 303, Diocletian ordered that the newly built church at Nicomedia be razed. He demanded that its scriptures be burned, and seized its precious stores for the treasury. The next day, Diocletian's first "Edict against the Christians" was published. The edict ordered the destruction of Christian scriptures and places of worship across the empire, and prohibited Christians from assembling for worship. Before the end of February, a fire destroyed part of the Imperial palace. Galerius convinced Diocletian that the culprits were Christians, conspirators who had plotted with the eunuchs of the palace. An investigation was commissioned, but no responsible party was found. Executions followed anyway, and the palace eunuchs Dorotheus and Gorgonius were executed. One individual, Peter Cubicularius, was stripped, raised high, and scourged. Salt and vinegar were poured in his wounds, and he was slowly boiled over an open flame. The executions continued until at least 24 April 303, when six individuals, including the bishop Anthimus, were decapitated. A second fire occurred sixteen days after the first. Galerius left the city for Rome, declaring Nicomedia unsafe. Diocletian would soon follow.
Although further persecutory edicts followed, compelling the arrest of the Christian clergy and universal acts of sacrifice, they were ultimately unsuccessful; most Christians escaped punishment, and pagans too were generally unsympathetic to the persecution. The martyrs' sufferings strengthened the resolve of their fellow Christians. Constantius and Maximian did not apply the later edicts, and left the Christians of the West unharmed. Galerius rescinded the edict in 311, announcing that the persecution had failed to bring Christians back to traditional religion. The temporary apostasy of some Christians, and the surrendering of scriptures, during the persecution played a major role in the subsequent Donatist controversy. Within twenty-five years of the persecution's inauguration, the Christian emperor Constantine would rule the empire alone. He would reverse the consequences of the edicts, and return all confiscated property to Christians. Under Constantine's rule, Christianity would become the empire's preferred religion. Diocletian was demonized by his Christian successors: Lactantius intimated that Diocletian's ascendancy heralded the apocalypse.