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 honor of a consulship in 283.

– left his sons Numerian and Carinus as the new Augusti.

Under this 'tetrarchy', or "rule of four", each emperor would rule over a quarter-division of the empire.

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. Diocletian led the subsequent negotiations and achieved a lasting and favorable peace.

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.

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.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.Carinus quickly made his way to Rome from his post in Gaul as imperial commissioner and arrived there by January 284, becoming legitimate Emperor in the West. The Sassanid king Bahram II could not field an army against them as he was still struggling to establish his authority.By March 284, Numerian had only reached Emesa (Homs) in Syria; by November, only Asia Minor.Diocletian separated and enlarged the empire's civil and military services and reorganized the empire's provincial divisions, establishing the largest and most bureaucratic government in the history of the empire.