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Early rule

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 Rome at this time. There is a contemporary issue of coins suggestive of an imperial adventus (arrival) for the city, but some modern historians state that Diocletian avoided the city, to demonstrate that the city and its Senate were no longer politically relevant to the affairs of the empire. Diocletian dated his reign from his elevation by the army, not his ratification by the Senate, following the practice established by Carus, who had declared the Senate's ratification a useless formality. However, Diocletian offered 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) and by creating senior senators Vettius Aquilinus and Junius Maximus ordinary consuls for the following year – for Maximus, it was his second consulship.

If Diocletian did enter Rome shortly after his accession, he did not stay long; he is attested back in the Balkans by 2 November 285, on campaign against the Sarmatians.

Diocletian replaced the prefect of Rome with his consular colleague Bassus. Most officials who had served under Carinus, however, retained their offices under Diocletian. In an act of clementia denoted by the epitomator of Aurelius Victor as unusual, Diocletian did not kill or depose Carinus's traitorous praetorian prefect and consul Aristobulus, but confirmed him in both roles. He later gave him the proconsulate of Africa and the post of urban prefect for 295. The other figures who retained their offices might have also betrayed Carinus.