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.The first forty years of his life are mostly obscure.
He appointed fellow officer Maximian as Augustus, co-emperor, in 286.
Diocletian delegated further on 1 March 293, appointing Galerius and Constantius as Caesars, junior co-emperors.
On 20 November 284, the army of the east gathered on a hill 5 kilometres (3.1 mi) outside Nicomedia.
The army unanimously saluted Diocles as their new augustus, and he accepted the purple imperial vestments.
Julianus minted coins from the mint at Siscia (Sisak, Croatia) declaring himself as emperor and promising freedom.
It was all good publicity for Diocletian, and it aided in his portrayal of Carinus as a cruel and oppressive tyrant.
Diocletian may have become involved in battles against the Quadi and Marcomanni immediately after the Battle of the Margus.
He eventually made his way to northern Italy and made an imperial government, but it is not known whether he visited the city of Rome at this time.
Building on third-century trends towards absolutism, he styled himself an autocrat, elevating himself above the empire's masses with imposing forms of court ceremonies and architecture.
Bureaucratic and military growth, constant campaigning, and construction projects increased the state's expenditures and necessitated a comprehensive tax reform.
Under this 'tetrarchy', or "rule of four", each emperor would rule over a quarter-division of the empire.