From ABC-CLIO's World History: Ancient and Medieval Eras website
Diocletian served as emperor of Rome from 284 to 305 CE. He is remembered as a shrewd administrator and for using a short-lived but effective system of government known as the tetrarchy.
Gaius Aurelius Valerius Diocletianus was born Diocles in 245 CE to an Illyrian family of modest status. He proved himself to be a brave solider and worked his way through the ranks of the Roman military. Following the murder of Emperor Numerianus, supposedly by the praetorian prefect Aper, whom Diocletian later executed, Diocletian was chosen by the army to become emperor in 284. After his accession, he changed his name from Diocles to Gaius Aurelius Valerius Diocletianus.
In one of his first acts as augustus, a title he adopted as emperor, Diocletian appointed Maximian as his heir and successor, or caesar, in 285. Recognizing that threats to the Roman Empire on two fronts, along with exceedingly difficult internal struggles, were too much to be controlled under one leader, Diocletian divided the Roman Empire in two along an invisible line that ran from the Danube River to Dalmatia. Diocletian then proclaimed that his caesar, Maximian, would rule over the western portion while he would reign in the east. One year later, in 286, Diocletian promoted Maximian to the rank of coemperor, or augustus, although he still retained ultimate sovereignty over his coemperor and the Roman Empire.
While the division of power between the two emperors made it easier for Diocletian and Maximian to rule the vast Roman Empire, the new arrangement served to marginalize the power of the Roman Senate. In addition, both Diocletian and Maximian ruled from separate co-capitals, neither of which was the city of Rome. Diocletian, in fact, moved his capital quite often in order to protect the government from both internal and external dangers.
A number of years later, in 292, each emperor was given a caesar, or successor—Diocletian's was Galerius, and Maximian's was Constantius I Chlorus—in an attempt to ensure that there would be an orderly transfer of power when the time came. Those two caesars were given the authority to act as more than successors, however, and were each permitted to rule over approximately one-quarter of the empire. That unique governmental arrangement became known as the tetrarchy. Although the tetrarchy worked fairly well for Diocletian and his colleagues, it soon fell victim to the opportunistic nature of Roman imperialism and was replaced again by one-man rule not long after Diocletian's abdication.
In addition to reorganizing imperial politics, Diocletian reorganized and strengthened the Roman Army as well. That reorganization resulted in two separate kinds of troops: the frontiersmen, or limitanei, who protected the borders; and the highly mobile central army known as the comitatensis, which stood prepared to attack or defend any location on short notice.
Diocletian also reorganized the administrative borders of the Roman Empire to break up the provinces into smaller units called prefectures and dioceses. Those territories were then ruled by civil prefects or governors, who kept control of local administration and tax collection, but who unlike their predecessors had no military authority. With those reforms, Diocletian kept potential rebellious proconsuls or prefects in check and prevented them from access to funds with which they might finance a rebellion. He also attempted to restore the gold standard and issued an edict in 301 aimed at regulating wages and prices.
Furthermore, Diocletian sought to resanctify the imperial seat during his reign, which left little room for the tolerance of religious worship apart from loyalty to the Roman Empire. It was near the end of Diocletian's rule that one of the most violent Roman-sponsored persecutions of Christians began. Reportedly, Diocletian was greatly influenced by his caesar, Galerius. As a result, Diocletian undertook a 10-year-long offensive against the Christians, who had enjoyed relative peace and prosperity prior to the appointment of Galerius. In approximately 302, Diocletian and Galerius presided over a council in Nicomedia that proclaimed that Christianity should be suppressed throughout the Roman Empire. That declaration was followed by a series of edicts ordering the destruction of churches and church property, which promptly took place. After Diocletian's retirement in 305, the persecution became even more violent and widespread.
In approximately 304, following Rome's long-fought victory against the Persians, Diocletian and Maximian celebrated the final triumph of Rome on the occasion of Diocletian's 20th year as emperor. The following year, in 305, Diocletian became the only Roman emperor to step down voluntarily from the seat of power. He then retired to his palace in Salona on the Adriatic Sea, where he died in 313. On his abdication, Maximian retired as well and settled in Luciana in southern Italy. Diocletian and Maximian were succeeded by their caesars, Galerius and Constantius, respectively.
Diocletian was survived by his wife, Prisca, and his daughter, Valeria, who in 293 became the second wife of Diocletian's caesar, Galerius.
Haber, Katharine. "Diocletian." World History: Ancient and Medieval Eras. ABC-CLIO, 2014. Web. 6 Jan. 2014.