Diocletian was Roman emperor from 284 to 305 CE. Born to a family of low status in the Roman province of Dalmatia, Diocletian rose through the ranks of the military to become cavalry commander to the Emperor Carus. After the deaths of Carus and his son Numerian on campaign in Persia, Diocletian was proclaimed emperor. Diocletian’s reign stabilized the empire, and marked the end of the Crisis of the Third Century. He appointed fellow officer, Maximian, as Augustus, co-emperor, in 286. Diocletian delegated further in 293, appointing Galerius and Constantius as caesars, junior co-emperors. Under this “tetrarchy,” or “rule of four,” each emperor would rule over a quarter-division of the empire. Diocletian further secured the empire’s borders and purged it of all threats to his power.
He 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, Antioch, and Trier, closer to the empire’s frontiers than the traditional capital at Rome had been. 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.
The first phase of Diocletian’s government restructuring, sometimes referred to as the diarchy (“rule of two”), involved the designation of the general Maximian as co-emperor—first as Caesar (junior emperor) in 285, then Augustus in 286. This reorganization allowed Diocletian to take care of matters in the eastern regions of the empire, while Maximian similarly took charge of the western regions, thereby halving the administrative work required to oversee an empire as large as Rome’s. In 293, feeling more focus was needed on both civic and military problems, Diocletian, with Maximian’s consent, expanded the imperial college by appointing two Caesars (one responsible to each Augustus)—Galerius and Constantius Chlorus.
In 305, the senior emperors jointly abdicated and retired, allowing Constantius and Galerius to be elevated in rank to Augusti. They in turn appointed two new Caesars—Severus II in the west under Constantius, and Maximinus in the east under Galerius—thereby creating the second tetrarchy.
The four tetrarchs based themselves not at Rome but in other cities closer to the frontiers, mainly intended as headquarters for the defense of the empire against bordering rivals. Although Rome ceased to be an operational capital, it continued to be the nominal capital of the entire Roman Empire, not reduced to the status of a province, but under its own, unique Prefect of the City (praefectus urbis).
In terms of regional jurisdiction, there was no precise division between the four tetrarchs, and this period did not see the Roman state actually split up into four distinct sub-empires. Each emperor had his zone of influence within the Roman Empire, but this influence mainly applied to the theater of war. The tetrarch was himself often in the field, while delegating most of the administration to the hierarchic bureaucracy headed by his respective Praetorian Prefect. The Praetorian Prefect was the title of a high office in the Roman Empire, originating as the commander of the Praetorian Guard, the office gradually acquired extensive legal and administrative functions, with its holders becoming the emperor’s chief aides.
Demise of the Tetrarchy
When, in 305, the 20-year term of Diocletian and Maximian ended, both abdicated. Their Caesares, Galerius and Constantius Chlorus, were both raised to the rank of Augustus, and two new Caesares were appointed: Maximinus (Caesar to Galerius) and Flavius Valerius Severus (Caesar to Constantius). These four formed the second tetrarchy.
However, the system broke down very quickly thereafter. When Constantius died in 306, Galerius promoted Severus to Augustus while Constantine, Constantius’ son, was proclaimed Augustus by his father’s troops. At the same time, Maxentius, the son of Maximian, who also resented being left out of the new arrangements, defeated Severus before forcing him to abdicate and then arranging his murder in 307. Maxentius and Maximian both then declared themselves Augusti. By 308, there were therefore no fewer than four claimants to the rank of Augustus (Galerius, Constantine, Maximian and Maxentius), and only one to that of Caesar (Maximinus).
In 308, Galerius, together with the retired emperor Diocletian and the supposedly retired Maximian, called an imperial “conference” at Carnuntum on the River Danube. The council agreed that Licinius would become Augustus in the West, with Constantine as his Caesar. In the East, Galerius remained Augustus, and Maximinus remained his Caesar. Maximian was to retire, and Maxentius was declared an usurper. This agreement proved disastrous: by 308 Maxentius had become de facto ruler of Italy and Africa even without any imperial rank, and neither Constantine nor Maximinus—who had both been Caesares since 306 and 305, respectively—were prepared to tolerate the promotion of the Augustus Licinius as their superior.
After an abortive attempt to placate both Constantine and Maximinus with the meaningless title filius Augusti (“son of the Augustus,” essentially an alternative title for Caesar), they both had to be recognized as Augusti in 309. However, four full Augusti all at odds with each other did not bode well for the tetrarchic system.
Between 309 and 313, most of the claimants to the imperial office died or were killed in various civil wars. Constantine forced Maximian’s suicide in 310. Galerius died naturally in 311. Maxentius was defeated by Constantine at the Battle of the Milvian Bridge in 312, and subsequently killed. Maximinus committed suicide at Tarsus in 313, after being defeated in battle by Licinius.
By 313, therefore, there remained only two emperors: Constantine in the west and Licinius in the east. The tetrarchic system was at an end, although it took until 324 for Constantine to finally defeat Licinius, reunite the two halves of the Roman Empire, and declare himself sole Augustus.