He lived out his retirement in his palace on the Dalmatian coast, tending to his vegetable gardens.
Although effective while he ruled, Diocletian's tetrarchic system collapsed after his abdication under the competing dynastic claims of Maxentius and Constantine, sons of Maximian and Constantius respectively.
The Diocletianic Persecution (303–11), the empire's last, largest, and bloodiest official persecution of Christianity, failed to eliminate Christianity in the empire; indeed, after 324, Christianity became the empire's preferred religion under its first Christian emperor, Constantine.
Augustus, the first Emperor, had nominally shared power with his colleagues, and more formal offices of Co-Emperor had existed from Marcus Aurelius on.
Most recently, Emperor Carus and his sons had ruled together, albeit unsuccessfully.
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 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.
Diocletian may have become involved in battles against the Quadi and Marcomanni immediately after the Battle of the Margus.