Diocletian
Conflict with Carinus

After his accession, Diocletian and Lucius Caesonius Bassus were named as consuls and assumed the fasces in place of Carinus and Numerian. Bassus was a member of a senatorial family from Campania, a former consul and proconsul of Africa, chosen by Probus for signal distinction. He was skilled in areas of government where Diocletian presumably had no experience. Diocletian's elevation of Bassus symbolized his rejection of Carinus' government in Rome, his refusal to accept second-tier status to any other emperor, and his willingness to continue the long-standing collaboration between the empire's senatorial and military aristocracies. It also tied his success to that of the Senate, whose support he would need in his advance on Rome.

Diocletian was not the only challenger to Carinus' rule; the usurper Julianus, Carinus' corrector Venetiae, took control of northern Italy and Pannonia after Diocletian's accession. Julianus minted coins from Siscia (Sisak, Croatia) declaring himself emperor and promising freedom. This aided Diocletian in his portrayal of Carinus as a cruel and oppressive tyrant. Julianus' forces were weak, and were handily dispersed when Carinus' armies moved from Britain to northern Italy. As the leader of the united East, Diocletian was clearly the greater threat. Over the winter of 284–85, Diocletian advanced west across the Balkans. In the spring, some time before the end of May, his armies met Carinus' across the river Margus (Great Morava) in Moesia. In modern accounts, the site has been located between the Mons Aureus (Seone, west of Smederevo) and Viminacium, near modern Belgrade, Serbia.

Despite having a stronger, more powerful army, Carinus held the weaker position. His rule was unpopular, and it was later alleged that he had mistreated the Senate and seduced his officers' wives. It is possible that Flavius Constantius, the governor of Dalmatia and Diocletian's associate in the household guard, had already defected to Diocletian in the early spring. When the Battle of the Margus began, Carinus' prefect Aristobulus also defected. In the course of the battle, Carinus was killed by his own men. Following Diocletian's victory, both the western and the eastern armies acclaimed him as Emperor. Diocletian exacted an oath of allegiance from the defeated army and departed for Italy.
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