Diocletian was born in the Roman province of Dalmatia, probably near the town of Salona (modern Solin in Croatia), where he eventually retired. His original name was Gaius Valerius Diocles, possibly derived from the name of his mother and her birthplace, Dioclea. His official birthday was 22 December, and based on later accounts that he died at about 68, he was likely born between 242 and 245. His parents were of low social status; some ancient writers say his father was a scribe, others that Diocles himself had once been a freedman of a senator called Anullinus. The first forty years of his life are poorly documented. We know that he was from the Illyrian regions and served as a soldier under the emperors Aurelian and Probus. Later sources claim he held high commands on the Danube frontier, but details of his early career remain uncertain. The first firmly attested point in his life is in 282, when Emperor Carus appointed him commander of the protectores domestici, an elite cavalry bodyguard. This position brought him enough prestige to become consul in 283.
After Carus died suddenly during a successful campaign against Persia—rumored to have been caused either by lightning or by enemy action—his sons Carinus and Numerian became emperors. Carinus took control of the West, ruling from Rome, while Numerian remained with the army in the East. During the return march from Persia, Numerian reportedly developed an eye disease and began traveling in a closed coach. When the army reached Bithynia, the soldiers noticed a foul smell coming from the coach, opened it, and discovered that Numerian was dead. The powerful court official Aper, Numerian’s father-in-law, announced the news in Nicomedia. The generals and tribunes gathered to choose a new emperor and selected Diocles. On 20 November 284, the army of the East met outside Nicomedia and hailed him as Augustus. In front of the assembled troops, Diocles swore that he was not responsible for Numerian’s death and accused Aper of murder. He then killed Aper with his own hand before the soldiers. Soon afterward, Diocles adopted the more Latinized name Gaius Valerius Diocletianus—Diocletian.
Diocletian’s first major challenge was the rival emperor Carinus, who still ruled in the West. Diocletian appointed an experienced senator, Lucius Caesonius Bassus, as his consular colleague, signaling a break with Carinus’ regime and an alliance with the Senate. At the same time, another usurper, Julianus, proclaimed himself emperor in northern Italy and Pannonia, minting coins and briefly complicating the political situation. Carinus defeated Julianus but then had to face Diocletian. In the spring of 285, their armies met on the river Margus in Moesia (in the Balkans). Although Carinus commanded a larger and stronger army, his rule was unpopular; there were accusations that he had mistreated the Senate and seduced the wives of his officers. During the battle, his prefect Aristobulus defected, and Carinus was ultimately killed by his own men. Diocletian emerged as sole emperor recognized by both the eastern and western armies, took their oath of loyalty, and marched toward Italy.
In the early years of his reign, Diocletian probably campaigned against Germanic tribes such as the Quadi and Marcomanni and consolidated his position in northern Italy. It is unclear whether he visited Rome immediately; if he did, he did not stay long. He preferred to rule from strategic provincial centers closer to the frontiers rather than from the traditional capital. Diocletian dated his reign from the day the army proclaimed him emperor, not from senatorial recognition, emphasizing that his power came from military acclamation, not the Senate. Nevertheless, he maintained a working relationship with the senatorial class, appointing prominent senators as consuls and retaining many officials who had served under Carinus. In a show of clemency unusual for that period, he even confirmed Aristobulus—who had betrayed Carinus—as praetorian prefect and later entrusted him with other high offices.
Recognizing that the empire was too vast and troubled for one man to rule effectively, Diocletian soon chose a colleague. In 285 he elevated his trusted fellow officer Maximian to the rank of Caesar, effectively making him junior co-ruler and heir, and soon afterward promoted him to Augustus, making them equal emperors. Together, they divided responsibilities: Diocletian took the East, Maximian the West. They strengthened their bond symbolically by adopting one another’s family names and by presenting themselves in religious terms: Diocletian associated himself with Jupiter (Iovius), the chief god and supreme authority, while Maximian associated himself with Hercules (Herculius), Jupiter’s loyal and powerful helper. This imagery reinforced a vision of cooperative rule in which Diocletian planned and commanded and Maximian acted as his heroic partner.
While Maximian struggled with revolts, including that of Carausius, who set himself up as a breakaway emperor in Britain and parts of northern Gaul, Diocletian focused on securing the Danube frontier and managing relations in the East. He fought Sarmatian and other tribes along the Danube, reorganized the frontier defenses, and fortified key cities. In the East, he took advantage of instability in the Sassanid Persian Empire. Through diplomatic and military pressure, he gained recognition of Roman control over parts of Armenia and Mesopotamia, strengthened the frontier, and earned the title “founder of eternal peace.” In Egypt, Diocletian faced serious rebellion after he attempted to reform taxes and administration. A usurper, Domitius Domitianus, seized control of Alexandria and much of the province. Diocletian personally led a campaign to reclaim Egypt, suppressed the revolt, besieged and captured Alexandria, and then reorganized the province, bringing its bureaucratic and fiscal practices more into line with the rest of the empire.
To stabilize government and succession more permanently, Diocletian created the Tetrarchy in 293. He and Maximian remained senior emperors (Augusti), but each appointed a junior emperor (Caesar): Galerius in the East and Constantius in the West. These four rulers each governed a portion of the empire, with their own courts, armies, and administrative centers, but they were bound by a carefully constructed network of family ties and formal adoptions. The system was meant to provide orderly succession: the Caesars would eventually become Augusti, while new Caesars would be chosen, ideally avoiding civil wars over the throne. The four emperors spent much of their time on campaign or dealing with local crises: Galerius fought Persians and Sarmatians, Constantius eventually defeated Carausius’ regime in Britain, and Diocletian continued to strengthen the Danube and eastern frontiers.
Diocletian was a traditionalist in religious matters and devoted to the old Roman gods. At first his policy toward Christians was relatively tolerant, but around 299–303 a shift occurred. An attempt at divination at court allegedly failed because Christian officials refused to participate in sacrifices. Influenced especially by Galerius and by oracular consultation, Diocletian ordered increasingly harsh measures. Christian soldiers and officials were required to sacrifice or lose their positions; then a series of edicts ordered the destruction of churches, the burning of scriptures, the arrest of clergy, and forced public sacrifices under pain of imprisonment, torture, or death. This period, known as the Great Persecution, was the most severe anti-Christian campaign in Roman history, though it was enforced unevenly: some regions, particularly in the West under Constantius, saw relatively mild application. In the long run, the persecution failed. Within a generation, Christianity would gain imperial favor under Constantine, and later Christian writers portrayed Diocletian as a villain for his role in these events.
In his later years, Diocletian’s health declined. After a taxing campaign on the Danube and a collapse during a public ceremony in Nicomedia, he spent months out of sight and was rumored to be dead. In 305, appearing visibly weakened, he did something unprecedented: he voluntarily abdicated the imperial throne. On 1 May 305, at the same hill near Nicomedia where he had once been proclaimed emperor, he formally laid down his powers. On the same day, Maximian also retired. The two Caesars, Galerius and Constantius, were promoted to Augusti, and two new Caesars—Severus and Maximinus Daia—were appointed. Notably, the adult sons of the Augusti, Constantine and Maxentius, were passed over, a decision that would later help destabilize the system.
Diocletian retired to his native Dalmatia, to a grand fortified palace he had built near Salona, at Spalatum (modern Split in Croatia). There he lived as a private citizen, tending his gardens and enjoying a quieter life far from the intrigues of court. When later emperors and generals urged him to return to power to resolve the civil conflicts that erupted after his retirement, he reportedly refused, saying that if they could see the cabbages he had grown with his own hands, they would never ask him to give up such peace for the storms of power. He lived to see the Tetrarchic system he designed collapse into a new round of civil wars, and to hear of the suicide and condemnation of his former colleague Maximian. Diocletian died in his palace in 311 or 312, possibly by his own hand, leaving behind a transformed empire.
His legacy rests largely on his reforms. Diocletian greatly expanded and reorganized the imperial bureaucracy, dividing the empire into many more provinces grouped into larger dioceses, each overseen by new layers of officials. He separated military and civil authority, giving military command to duces and comites, while governors handled justice and taxes. He strengthened frontiers, especially along the Danube and in the East, and reorganized imperial finances and tax systems. Ideologically, he abandoned the old fiction that the emperor was merely “first among equals,” instead presenting the emperor as a distant, sacred monarch, surrounded by ceremony, jeweled robes, and strict protocol. Though many of his arrangements unraveled after his retirement, Diocletian’s reordering of the state laid much of the groundwork for the later, more centralized and militarized Roman Empire of Late Antiquity.
