AE Antoninianus, A.D. 284-294, Cyzicus, 21.7mm, 4.56g, 180°, RIC V 306.
Obv: IMP C C VAL DIOCLETIANVS AVG. Radiate, draped and cuirassed bust right.
Rev: CONCORDIA MILITVM. Emperor and Jupiter standing. Jupiter holding globe with Victory; Γ in field. XXI• in ex.
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Born to an Illyrian family of low status in the Roman province of Dalmatia, Diocletian rose through the ranks of the military to become cavalry commander to the emperor Carus. After the deaths of Carus and his son Numerian on campaign in Persia, Diocletian was proclaimed emperor. The title was also claimed by Carus' other surviving son, Carinus, but Diocletian defeated him in the Battle of the Margus. With his accession to power, Diocletian ended the Crisis of the Third Century. He appointed fellow officer Maximian his senior co-emperor and also appointed Galerius and Constantius as Caesars. In this new "Tetrarchy", or "rule of four", each emperor would rule over a quarter-division of the empire.

Diocletian secured the empire's borders and purged it of all threats to his power. He defeated the Sarmatians and Carpi during several campaigns between 285 and 299, the Alamanni in 288, and usurpers in Egypt between 297 and 298. Galerius, aided by Diocletian, campaigned successfully against Sassanid Persia, the empire's traditional enemy. In 299 he sacked their capital, Ctesiphon. Diocletian led the subsequent negotiations and achieved a lasting and favorable peace. The Diocletianic Persecution (303–11), the empire's last, and bloodiest official persecution of Christianity, did not destroy the empire's Christian community; indeed, after 324 Christianity became the empire's preferred religion under its first Christian emperor, Constantine. In spite of several failures, Diocletian's reforms helped to stabilize the empire economically and militarily, enabling the empire to remain essentially intact for another hundred years despite being near the brink of collapse in Diocletian's youth.

Weakened by illness, Diocletian left the imperial office on May 1, 305, and became the only Roman emperor to voluntarily abdicate the position. He lived out his retirement in his palace on the Dalmatian coast, tending to his vegetable gardens. His palace eventually became the core of the modern-day city of Split.