DIOCLETIAN (Gaius Aurelius Valerius Diocletianus) (a.d. 245-313), Roman emperor 284-305, is said to have been born at Dioclea, near Salona, in Dalmatia. His original name was Diocles. Of humble origin, he served with high distinction and held important military commands under the emperors Probus and Aurelian, and accompanied Carus to the Persian War. After the death of Numerianus he was chosen emperor by the troops at Chalcedon, on the 17th of September 284, and slew with his own hands Arrius Aper, the praefect of the praetorians. He thus fulfilled the prediction of a druidess of Gaul, that he would mount a throne as soon as he had slain a wild boar (aper). Having been installed at Nicomedia, he received general acknowledgment after the murder of Carinus. In consequence of the rising of the Bagaudae in Gaul, and the threatening attitude of the German peoples on the Rhine, he appointed Maximian Augustus in 286; and, in view of further dangers and disturbances in the empire, proclaimed Constantius Chlorus and Galerius Caesars in 293. Each of the four rulers was placed at a separate capital - Nicomedia, Mediolanum (Milan), Augusta Trevirorum (Trier), Sirmium. This amounted to an entirely new organization of the empire, on a plan commensurate with the work of government which it now had to carry on. At the age of fifty-nine, exhausted with labour, Diocletian abdicated his sovereignty on the 1st of May 305, and retired to Salona, where he died eight years afterwards (others give 316 as the year of his death). The end of his reign was memorable for the persecution of the Christians. In defence of this it may be urged that he hoped to strengthen the empire by reviving the old religion, and that the church as an independent state over whose inner life at least he possessed no influence, appeared to be a standing menace to his authority. Under Diocletian the senate became a political nonentity, the last traces of republican institutions disappeared, and were replaced by an absolute monarchy approaching to despotism. He wore the royal diadem, assumed the title of lord, and introduced a complicated system of ceremonial and etiquette, borrowed from the East, in order to surround the monarchy and its representative with mysterious sanctity. But at the same time he devoted his energies to the improvement of the administration of the empire; he reformed the standard of coinage, fixed the price of provisions and other necessaries of daily life, remitted the tax upon inheritances and manumissions, abolished various monopolies, repressed corruption and encouraged trade. In addition, he adorned the city with numerous buildings, such as the thermae, of which extensive remains are still standing (Aurelius Victor, De Caesaribus, 39; Eutropius ix. 13; Zonaras xii. 31).
See A. Vogel, Der Kaiser Diocletian (Gotha, 1857), a short sketch, with notes on the authorities; T. Preuss, Kaiser Diocletian und seine Zeit (Leipzig, 1869); V. Casagrandi, Diocleziano (Faenza, 1876); H. Schiller, Gesch. der römischen Kaiserzeit, ii. (1887); T. Bernhardt, Geschichte Roms von Valerian bis zu Diocletians Tod (1867); A. J. Mason, The Persecution of Diocletian (1876); P. Allard, La Persécution de Dioclétien (1890); V. Schultze in Herzog-Hauck's Realencyklopädie für protestantische Theologie, iv. (1898); Gibbon. Decline and Fall, chaps. 13 and 16; A. W. Hunzinger, Die Diocletianische Staatsreform (1899); O. Seeck, "Die Schatzungsordnung Diocletians" in Zeitschrift für Social- und Wirthschaftsgeschichte (1896), a valuable paper with notes containing references to sources; and O. Seeck, Geschichte des Untergangs der antiken Welt, vol. i. cap. 1. On his military reforms see T. Mommsen in Hermes, xxiv., and on his tariff system, Diocletian, Edict of.
Note - this article incorporates content from Encyclopaedia Britannica, Eleventh Edition, (1910-1911)