Diocletian (245-ca. 313), in full, Gaius Aurelius Valerius Diocletianus, was a Roman emperor. He established the characteristic form of government for the later empire, the Dominate.

Diocletian, whose name before he became emperor was simply Diocles, was a Dalmatian of humble birth. He became commander of Emperor Numerian's bodyguard. When the Emperor was murdered by his praetorian prefect, the troops chose Diocletian in November 284 to succeed and avenge his master.

By early 285 Diocletian had circumvented all opposition and determined to take immediate steps to bring to an end the 50 years of military anarchy (235-284) that had seen 26 emperors gain the throne, and scores of unsuccessful pretenders. He therefore decided to appoint as his caesar (successor-designate) a man of his own age, his old fellow soldier Maximian. The wisdom of this policy was immediately demonstrated by Maximian's military successes in Gaul, Germany, and North Africa between 286 and 290. Diocletian, meanwhile, controlled...