Septimius Severus had to deal with Pescennius Niger, who didn't accept his claim as emperor. In 193-194, Niger was defeated and the Roman Empire united again.
Niger was supported by king Vologases V of the Parthian Empire, who managed to reconquer Roman Mesopotamia. In 195, the Parthians were defeated and Nisibis became Roman again.
After the Parthians were defeated, Septimius Severus named his son Caracalla as his successor. Clodius Albinus who thought that he would be the successor, feared for his life and revolted, invading Gaul in 196.
A major revolt of Celts and Picts in Britain completely wiped out Roman pressence, while Albinus was in Gaul. Governor Lucius Novius Rufus of Hispania Tarraconensis came to the aid of Albinus. The Rhine armies, commanded by Virius Lupus continued to support Septiumius Severus, even after their defeat. Albinus was defeated at the Battle of Lugdunum in 197.
After the defeat of Albinus, Septimius Severus turned his attention again to the Parthians, still in 197. He also invaded Mesopotamia and sacked the Parthian capital Ctesiphon. Roman Mesopotamia was enlarged, from a garrison in Nisibis to an area with fully garrisoned borders.
After defeating Albinus, Septimius Severus send Verus to Britain as the new governor. Verus had a hard time controlling the British tribes. Having to buy off the Maeatae. Hadrian's Wall could only be rebuild in 205. Septimius Severus himself came to Britain to root out anti-Roman rebellion. He primarily campaigned above Hadrian's Wall and also above the Antonine Wall against the Maeatae (probably southern Picts) and the Caledonian Confederacy (probably northern Picts). Peace was made in 210 with the Picts. Septimius Severus died in 211 in Eburacum, while his son Caracalla conducted a military campaign versus the Maeatae.