After Probus’ successor Carus (282–3) died on campaign in Persia, the popular general Diocles was proclaimed emperor by the eastern legions. Taking the name ‘Diocletian’, the new emperor marched west to defeat Carus’ son Carinus at Margum.
Receiving news of the revolts in Gaul, Probus made a truce with the Persians and returned west, resettling a large number of Bastarnae who submitted to him in Moesia or Thrace. Meanwhile, some Franks that he had also resettled rose up in revolt and gained control of a fleet of ships. After raiding Greece and Syracuse, and suffering a repulse outside Carthage, they successfully sailed back to their homeland on the Rhine Delta.
in wikipedia
Having learned that Persia was in civil war in late 282, Carus marched rapidly to the East and invaded Mesopotamia via Armenia in spring 283. The Romans successfully stormed Ctesiphon and Seleucia but, in around July 283, Carus fell ill. The emperor was found dead some days later, apparently after lightning had struck his tent, prompting his son Numerian to abandon the campaign.
in wikipedia
Upon the discovery of the death of Numerian, an assembly and tribunal were organized, in which the low-born but popular general Diocles denounced the praetorian prefect Aper as the murderer and personally ran him through with his sword. Diocles was then proclaimed emperor by the troops, adopting the more Latinate name “Diocletian”. Despite his claim to be the avenger of Numerian, Diocletian rejected the rule of Carus’ other son Carinus and marched west to depose him.
in wikipedia
During the Crisis of the Third Century, the Baquates and other Berber tribes became increasingly aggressive, forcing more and more concessions from the Roman inhabitants of southern Mauretania Tingitana. This came to an end in around 284–285, when, for unknown reasons, Roman authorities evacuated Volubilis and other cities south of the Loukkos river to fall back on Tingis (Tangier). Although the region would never be retaken by the Romans, Diocletian eventually secured land communications between Mauretania Tingitana and Mauretania Caesariensis by forming an alliance with the Baquates.
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In July 285 the Roman emperor Carinus advanced to meet the usurper Diocletian near Margum, on the Margus river, in Upper Moesia. Despite having superior numbers and apparently coming close to winning the battle, the emperor was abandoned by his own troops and defeated. Carinus died soon afterwards, possibly murdered by one of his own officers when he took refuge in the Danube fort of Cornacum (Sotin, Croatia), leaving Diocletian as sole ruler of the Roman Empire.
in wikipedia
