Carinus left Rome at once and set out for the east to meet Diocletian. On his way through Pannonia, he put down the usurper Sabinus Julianus and in July 285 he encountered the army of Diocletian at the Battle of the Margus River (the modern Morava River) in Moesia.
Historians differ on what then ensued. At the Battle of the Margus, according to one account, the valor of his troops had gained the day, but Carinus was assassinated by a tribune whose wife he had seduced. Another account represents the battle as resulting in a complete victory for Diocletian and claims that Carinus' army deserted him. This account may be confirmed by the fact that Diocletian kept in service Carinus' Praetorian Guard commander, Titus Claudius Aurelius Aristobulus.
Conflict boiled in every province, from Gaul to Syria, Egypt to the lower Danube. It was too much for one person to control, and Diocletian needed a lieutenant. At some time in 285 at Mediolanum (Milan), Diocletian raised his fellow-officer Maximian to the office of caesar, making him co-emperor.