Diocletian (Caius Aurelius Valerius Diocletianus)dī´əklē´shən [key], 245–313, Roman emperor (284–305), b. near Salona, Dalmatia (the modern Split, Croatia). Of humble birth, he obtained high military command under Probus and Aurelian and fought under Carus in Persia. The army proclaimed him emperor after the death of Numerian, and he became sole ruler when Carinus, joint emperor with Numerian, was murdered by his own officers. In order to repel the Germans he appointed Maximian augustus (286) and Constantius I and Galerius caesars (293). The four rulers had their respective capitals at Nicomedia, Mediolanum (modern Milan), Treveri (modern Trier), and Sirmium. In Diocletian's reign Britain was restored (296) to the empire, the Persians were subjugated (298), and the Marcomanni were expelled from the empire. Diocletian was the first to divide the empire formally and to set up a genuine autocracy with no theoretical checks. The Roman senate became a municipal council and all vestiges of Republican institutions disappeared. His economic reforms included an attempt to restore the gold standard and the Edict of Diocletian (301), an economic measure to regulate prices and wages. Its effects, however, proved ruinous to agriculture and the markets. The persecution of the Christians in the latter part of his reign was a course to which he had been instigated by Galerius. Diocletian abdicated (305), and Maximian resigned at the same time. Diocletian retired to his castle at Salona, from which he saw his system fail as his successors began to quarrel among themselves.
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