Diocletian ( DY-ə-KLEE-shən; Latin: Gaius Aurelius Valerius Diocletianus; Ancient Greek: Διοκλητιανός, romanized: Diokletianós; 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. As with other Illyrian soldiers of the period, Diocles rose through the ranks of the military early in his career, serving under Aurelian and Probus, and 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 initiated the process of permanently splitting the Roman Empire, appointing fellow officer Maximian as Augustus, co-emperor, in 286. Maximian ruled the Western Empire, while Diocletian reigned in the Eastern Empire as the senior Augustus. In 293, Diocletian delegated further, 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 Treverorum, 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 (301), 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 due to 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 was born in Dalmatia, probably at or near the town of Salona (modern Solin, Croatia), to which he retired later in life. His original name was Diocles (in full, Gaius Valerius Diocles), possibly derived from Dioclea, the name of both his mother and her supposed place of birth. Diocletian's 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 (his death year is not certain). His parents were of low status; Eutropius records "that he is said by most writers to have been the son of a scribe, but by some to have been a freedman of a senator called Anullinus." The first forty years of his life are mostly obscure. Diocletian was an Illyricianus who served under Aurelian and Probus. According to some scholars Diocles was trained and promoted by Aurelian and Probus, according to other scholars there is no reliable evidence of Diocles's early career. The 12th-century Byzantine chronicler Joannes Zonaras states that he was Dux Moesiae, a commander of forces on the lower Danube. The often-unreliable Historia Augusta states that he served in Gaul, but this is not corroborated by other sources and is ignored by modern historians. The first time Diocletian's whereabouts are accurately established was in 282 when the Emperor Carus made him commander of the Protectores domestici, the elite cavalry force directly attached to the Imperial household. This post earned him the honor of a consulship in 283.
Carus's death, amid a successful war with Persia and in mysterious circumstances – he was believed to have been struck by lightning or killed by Persian soldiers – left his sons Numerian and Carinus as the new Augusti. Carinus quickly made his way to Rome from his post in Gaul and arrived there by January 284, becoming the legitimate Emperor in the West. Numerian lingered in the East. The Roman withdrawal from Persia was orderly and unopposed. The Sassanid king Bahram II could not field an army against them as he was still struggling to establish his authority. By March 284, Numerian had only reached Emesa (Homs) in Syria; by November, only Asia Minor. In Emesa he was apparently still alive and in good health: he issued the only extant rescript in his name there, but after he left the city, his staff, including the prefect Aper (Numerian's father-in-law and the dominant influence in his entourage), reported that he suffered from an inflammation of the eyes. He traveled in a closed coach from then on. When the army reached Bithynia, some of the soldiers smelled an odor emanating from the coach. They opened its curtains and found Numerian dead. Both Eutropius and Aurelius Victor describe Numerian's death as an assassination.
Aper officially broke the news in Nicomedia (İzmit) in November. Numerian's generals and tribunes called a council for the succession, and chose Diocles as Emperor, in spite of Aper's attempts to garner support. On 20 November 284, the army of the east gathered on a hill 5 kilometres (3.1 mi) outside Nicomedia. The army unanimously saluted Diocles as their new Augustus, and he accepted the purple imperial vestments. He raised his sword to the light of the sun and swore an oath disclaiming responsibility for Numerian's death. He asserted that Aper had killed Numerian and concealed it. In full view of the army, Diocles drew his sword and killed Aper. Soon after Aper's death, Diocles changed his name to the more Latinate "Diocletianus" – in full, Gaius Valerius Diocletianus.
After his accession, Diocletian and Lucius Caesonius Bassus were named as consuls and assumed the fasces in place of Carinus and Numerian. Bassus was a member of a senatorial family from Campania, a former consul and proconsul of Africa, chosen by Probus for signal distinction. He was skilled in areas of government where Diocletian presumably had no experience. Diocletian's elevation of Bassus symbolized his rejection of Carinus' government in Rome, his refusal to accept second-tier status to any other emperor, and his willingness to continue the long-standing collaboration between the empire's senatorial and military aristocracies. It also tied his success to that of the Senate, whose support he would need in his advance on Rome.