Severus was appointed governor of Pannonia Superior by Emperor Commodus

Severus was appointed governor of Pannonia Superior by Emperor Commodus in 191, on the advice of Quintus Aemilius Laetus, prefect of the Praetorian Guard. The following year, Commodus was assassinated. Pertinax was a popular emperor who was assassinated by the Praetorian Guard in early 193. In response to Pertinax's assassination, Severus' legion XIV Gemina proclaimed him emperor at Carnuntum. Nearby legions like X Gemina at Vindobona quickly followed suit. Severus rushed to Italy after assembling an army.

Didius Julianus, Pertinax's successor in Rome, had won the emperorship in an auction. Julianus was executed after the Senate sentenced him to death. Severus took possession of Rome without opposition. He executed Pertinax's assassins and dismissed the rest of the Praetorian Guard, replacing them with loyal troops from his own legions.

Syria's legions had crowned Pescennius Niger's emperor. At the same time, Severus thought it was reasonable to grant the status of Caesar to Clodius Albinus, the powerful governor of Britannia who had undoubtedly supported Didius against him. With his rear guard secure, he advanced to the east and defeated Niger's army at the Battle of Issus (194). During his battle against Byzantium, he had the grave of his fellow-Carthaginian Hannibal covered in exquisite marble.

He spent the next year suppressing Mesopotamia and other Parthian vassals who had supported Niger. Following that, Severus named his son Caracalla as his heir, prompting Albinus to be crowned emperor by his men and conquer Gaul. Severus traveled north to meet him after a brief stay in Rome. Severus defeated and killed Clodius Albinus in the Battle of Lugdunum on February 19, 197, with an army of around 75,000 soldiers, mostly formed of Pannonian, Moesian, and Dacian legions and a significant number of auxiliaries.