The Emperor Commodus was assassinated on New Year's Eve 192 AD. Following his death, the assassins appointed the respected statesman and distinguished military commander Pertinax to fill his place. However, Pertinax too was disliked by the praetorian guard and was murdered in March of the following year. Didius Julianus bribed the praetorians and was thus made emperor. Soon, however, the frontier legions began to declare for their own candidates: Clodius Albinus in Britain, Septimius Severus on the Danube, and Pescennius Niger in the east. Severus marched on Rome and killed Didius Julianus and eventually defeated his rivals and four years later found himself undisputed ruler. This later became known as the "Year of the Six Emperors".
Severus fought major wars in Britain and the east. In 195 AD and 197-198 AD, he fought two separate campaigns against the Parthians. He created two new provinces, Mesopotamia and Osrheone, in the Parthian borderlands beyond the Euphrates, as well as Numidia in Africa. He also made changes in the treatment of the Roman army, raising new legions and improving the soldiers' pay and conditions for the first time since Domitian. In addition, Severus built a series of impressive new public buildings in his birthplace, Lepcis Magna in North Africa.
During the rule of the Severans, there was a marked increase in the importance of the provinces. Politically, Italians were becoming steadily less important as provincials took more and more of the key positions. The influence of the Senate fell and Italy began to be treated as simply another province. Rome was still important, and it still stood as the main city of the empire during this time.
Severus died at York in 211 AD, and was followed by his sons Marcus Antoninus (nicknamed Caracalla) and Geta. Caracalla had Geta murdered after only a few months of joint rule. He increased the pay of the soldiers and granted Roman citizenship to all the free male inhabitants of the empire. In 215 AD, Caracalla massacred the young male population of Alexandria for no apparent reason. He fought two major wars against the Parthians, but was murdered by a man with a private grievance before the end of the second.
Caracalla's successor, Macrinus, was the first non-senator to become emperor. His reign was ineffectual and short. In 218 AD, the Syrian legions restored the Severan family to power by appointing Caracalla's niece's fourteen-year-old son Elagabalus. Elagabalus proved to be an embarassment to the empire, and the real power resided with his mother and grandmother, and he was eventually replaced by his more acceptable cousin Severus Alexander.
It was during this period in Roman history that the decline of the empire truly began. The Parthian empire was dominating the Near East, and eastern power waas spreading. In 230 AD, the Persian ruler Ardashir invaded Roman territory, forcing Severus Alexander to stage a counter-attack. In 235 AD, Alexander came to terms with the Germans on the Rhine frontier rather than fight them, his soldiers became fed up and mudered Alexander and his mother, appointing Maximinus the Thracian as emperor.
The army had become more and more important during the reign of the Severans. The huge part played by the military during this period is demonstrated by the rise to power of Maximinus, who was neither a senator nor an equestrian, but an ordinary soldier who had risen through the ranks. Maximinus spent most of his time with the army, totally ignoring Rome. He was murdered while besieging the rebellious city of Aquileia in 238 AD.
The empire continued to decline as emperor followed emperor. Rome was being attacked from all sides. In 260 AD, the Emperor Valerian was captured by the Persians and died in captivity. His son Gallienus was forced to rely on help from the rulers of Palmyra. The western provinces broke away to form a separate Gallic empire. However, the Illyrian emperors (a series of soldier-emperors of Balkan origin) slowly put the empire to rights. Claudius II defeated the Goths, Aurelian suppressed the breakaway Palmyrene and Gallic realms, reuniting the empire. Carus invaded Mesopotamia and sacked Ctesiphon. But the cost of these wars and the invasions was not low and the empire never fully recovered.