Battle of the Margus

The Battle of the Margus was fought in July 285 between the armies of Roman Emperors Diocletian and Carinus in the valley of the Margus River (today Great Morava) in Moesia (present day Serbia). Carinus led the larger force, but the loyalty of this army was definitely questionable. Carinus had allegedly alienated men whose support his success depended upon, including mistreating the Senate and its womenfolk and seducing the wives of his officers. The exact circumstances of the battle are in doubt, but it is known for certain that Carinus was killed in the course of the battle, most probably by one of his own officers.

Diocletian was then left in sole control of the Roman Empire. The tide of the battle may have tilted to Carinus at first, only to shift in Diocletian's favor after the defection of Carinus' Praetorian Prefect, Aristobulus. Some scholars suspect that Aristobulus was the officer responsible for the murder of Carinus, an argument that gains credibility in the fact that Diocletian afterward rewarded Aristobulus by confirming him in office as Praetorian Prefect and Consul for the remainder of 285.

Aristobulus' Rationale
Although the battle decisively decided the issue of Roman leadership in Diocletian's favor, it is likely that, with the eastern provinces behind him, Diocletian would eventually have outlasted Carinus even if he had found himself on the losing side at Margus. After his victory, Diocletian administered the oath of loyalty to Carinus' former troops, then turned his attention to the Danube frontier where the Marcomanni and Quadi were conducting raids across the border.