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.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 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.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.
Diocletian dated his reign from his elevation by the army, not the date of his ratification by the Senate, However, Diocletian was to offer proof of his deference towards the Senate by retaining Aristobulus as ordinary consul and colleague for 285 (one of the few instances during the Late Empire in which an emperor admitted a privatus as his colleague) The concept of dual rulership was nothing new to the Roman Empire.
Under this 'tetrarchy', or "rule of four", each emperor would rule over a quarter-division of the empire.
Diocletian secured the empire's borders and purged it of all threats to his power.
Born to a family of low status in Dalmatia, Diocletian rose through the ranks of the military to become Roman cavalry commander to the Emperor Carus.
After the deaths of Carus and his son Numerian on campaign in Persia, Diocletian was proclaimed emperor.On 20 November 284, the army of the east gathered on a hill 5 kilometres (3.1 mi) outside Nicomedia.