Diocletian (AD 245–313)

Roman emperor 284–305 who initiated severe persecution of Christians in 303. He was commander of the protectores domestici (Roman staff officers) under the emperor Numerian, and proclaimed emperor by his troops following Numerian's death. He defeated his rival Carinus in 285. In 293 he appointed Maximian (c. 240–c. 310) as co-ruler and reorganized and subdivided the empire, with two joint and two subordinate emperors. This was known as the Tetrarchic system. In 305 he abdicated in favour of Galerius, living in retirement until his death.

During his reign Diocletian fought a successful war against the Persians, defeated Achilleus in Egypt, and subdued Carausius and Allectus in Britain. Diocletian also reformed the army, creating or reorganizing many of the units that would form the core of the later, much larger comitatenses, or field armies.