Severus's first course of action was to disband the Praetorian Guard and replace it with 15,000 Danubian legionnaires loyal to him. Then he set out to deal with Gaius Pescennius Niger, who was still proclaimed emperor by the Syrian legions. Before setting out for the east, Severus secured his western flank with an alliance to a potential rival, Clodius Albinus legate of Britain.

Niger's forces where spread out between Syria and the city of Byzantium. Severus blockaded the coast then divided Niger's army by landing in Asia minor and scoring two victories. Niger then retreated to Syria where he was defeated decisively in spring, 194. The rest of Niger's forces held out in Byzantium for two more years, until the city was finally starved out and subsequently sacked.

In the mean time, Albinus had grown weary of the alliance and proclaimed himself Emperor with the support of a large number of senators. He crossed the English Channel, occupied Gaul with his headquarters at Lugdunum. Severus reacted by proclaimed his son Bassianus as the rightful Caesar (a position originally promised to Albinus) then rushing off to west. On February 197, after a closely fought battle near Lyons, Albinus committed suicide. Severus followed up their victory by destroying the wealthiest city in the west, Lugdunum.

After purging the empire of Albinus's former supporters, Severus was once again forced to turn eastward when news of a Parthian invasion reached Rome. King Vologases had been held up in a long siege of the Roman stronghold of Nisbis in northern Mesopotamia. When Severus arrived late in 197, King Vologases raised the siege and retreated. The Roman legions went on to sack Selucia and the capital of Ctesiphon after a long siege. This defeat weakened the Parthian dynasty to such a degree that it caused an internal revolution that brought the highly aggressive house of Sassan to throne of Persia. Septimus Severus had inadvertently put the eastern border in more dangerous situation.

Later, Severus campained in Britian against the legendery Scottish hero Fingal. During the end of the campain Severus died at York. He left the empire to his sons and his last words to them was "make the soldiers rich and forget about everything else".