The first forty years of his life are mostly obscure.
The first time Diocletian's whereabouts are accurately established, in 282, he was made by the newly Emperor Carus commander of the Protectores domestici, the élite cavalry force directly attached to the Imperial household – a post that earned him the honour of a consulship in 283.
His palace eventually became the core of the modern-day city of Split in Croatia.
Diocles' parents were of low status, and writers critical of him claimed that his father was a scribe or a freedman of the senator Anullinus, or even that Diocles was a freedman himself.
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.
After the deaths of Carus and his son Numerian on campaign in Persia, Diocletian was proclaimed emperor.
Diocletian delegated further on 1 March 293, appointing Galerius and Constantius as Caesars, junior co-emperors, under himself and Maximian respectively.
Under this 'tetrarchy', or "rule of four", each emperor would rule over a quarter-division of the empire.
He asserted that Aper had killed Numerian and concealed it.
Julianus minted coins from the mint at Siscia (Sisak, Croatia) declaring himself as emperor and promising freedom.