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.
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 in Croatia.
Under this 'tetrarchy', or "rule of four", each emperor would rule over a quarter-division of the empire.
Diocletian delegated further on 1 March 293, appointing Galerius and Constantius as Caesars, junior co-emperors, under himself and Maximian respectively.
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 raised his sword to the light of the sun and swore an oath disclaiming responsibility for Numerian's death.
He asserted that Aper had killed Numerian and concealed it.