"The contemporaries of Severus, in the enjoyment of the peace and glory of his reign, forgave the cruelties by which it had been introduced. Posterity, who experienced the fatal results of his maxims and example, justly considered him as the principle author of the decline of the Roman Empire"
Edward Gibbon, The Decline and Fall of the Roman Empire
Gibbon's perception of Septimius Severus was based on his own view of Roman history- he wrote a great longitudinal study of Rome's fall, from the age of the Antonines to the age of the Florentines and in his survey he noted the chronological passing of power. For Gibbon and for many before and after him, Rome's history took an upward turn in the second century AD. The hereditary Julio-Claudian and Flavian dynasties with their dynastic freaks (Tiberius, Nero, Domitian) gave way to the meritocratic series of adopted emperors- Nerva (96-8), Trajan (98-117), Hadrian (117-38), Antoninus Pius (138-61) and Marcus Aurelius (161-80). After Aurelius though the Empire slipped back- Aurelius's son Commodus was awful and he was succeeded by a period of civil war (193-97), only brought to an end when Septimius Severus seized control and reigned for a period of years until 211. After Septimius there was chaos as well- as challengers for the empire rose and fell and in the end internal chaos led to external danger- with emperors dying on foreign frontiers and various parts of the empire splitting off. The only Emperor who seemed to survive for a great period of time amidst the chaos was Severus, and hence Gibbon blamed him for the later decline.
Severus's biographer Anthony Birley takes a more lateral approach than Gibbon and stresses the ways in which Severus was a creature of his times. Severus was born in Lepcis Magna to a family with strong links to Rome- as part of the Empire's evolution more and more provincial citizens were using power within Rome itself- the great Senatorial aristocracy had been wiped out in the 1st Century AD and was replaced by a new aristocracy from the provinces, particularly Africa. Severus's ancestors- his grandfather in particular- was part of this and probably knew great literary figures such as Tacitus and Pliny. Severus's reign though marked a new turning point: he was the first Emperor not to have been brought up in Rome. He felt no great affection for the city- spending only three years of his reign in Italy (possibly less) and spending most of his time out on campaign. In that sense he marked the beggining of an evolution from an Italian Roman principate- to one which resided at key points on the frontier- as with Diocletian at Nicomedia and Milan, or with Constantine's successors at Constantinople and Ravenna.
Severus himself was part of a rich civilisation. He was a contemporary of Galen, the great doctor, and Tertullian, the Christian saint. Aurelius of course was not merely an Emperor but a philosopher. Severus was lucky in his historian, Cassius Dio, who compiled a pretty extensive history of his reign upon which much of Birley's work is based. But within that civilisation there were debates about strategy- some beleived as with Hadrian in an empire which withdrew to and solidified its boundaries, some like Trajan and Aurelius believed in extending the imperial sway to conquer new territories. Severus stood in the second camp- he looked in particular to Marcus Aurelius as a model for his reign- attempting to extend the Roman empire's sway in the East, where he sought to add Mesopotamian territory to reinforce the exposed province of Syria, in the south he forced the Roman border in Africa further south towards the Garamantes and in Britain, he attempted the conquest of Scotland but died before he could accomplish it. Such advances needed reorganisation. Severus was one of the leaders of a military reorganisation- that again was going to be paralleled later. The early Roman emperors relied upon provincial armies and a small Praetorian Guard in the Capital- Severus called up three legions to become a mobile reserve and attempted to introduce more fluidity into the army. Using it as a fluid weapon of offence and response instead of a passive defensive force- that meant that he spent far more upon the military than his predecessors. The instability of 193-7 also forced him to raise the soldiers' pay at a rate which the sterner predecessors would never have done.
Returning to Gibbon's question then- because it is worth answering, why did these trends lead to a temporary collapse and did they contribute to the decline and fall of the Roman Empire? Lets instead of answering Gibbon, answer a different question- did Severan reforms lead in part to the problems of the third century? To that question I think we can answer an unambiguous yes- the Severan reforms led to massive inflation throughout the empire, they led to military instability with soldiers desiring increased pay and receiving it from every new contender, Severus and his predecessors contributed to smashing up the old Roman system and the replacement was painful. But equally whilst Severan reforms contributed to the mid third century collapse, they also contributed to the recovery in the late third century. It is no accident that Diocletian and his successors used Severan ideas to reanimate the Empire: the Roman Empire in these years could in part be seen as a society going through the shock of reorganisation- and going through it spectacularly because the symptoms of that shock were civil war and invasion. Severan reforms though- introducing the itinerant, provincial, non-Roman Emperor with a mobile force behind him- shaped the later Roman Empire. Those reforms went back into the reigns of Aurelius and beyond- but they formed the template of what the Empire looked like under Constantine.
Perhaps what this demonstrates is that whereas there was definitely a fall of the Roman Empire in the West- decline isn't always the best way of conveying what happened to Rome. Rome evolved from a Republic to a Principate, from a Principate to an Empire- the changes meant that the form of the state changed- at times those changes could be painful but often they were attempts to respond to actual situations. Of course there were important failures- as you would expect with any system that relied on one individual, selected at times by the random chance of their genes, and Rome seemed to specialise in competent fathers with useless sons (Severus's son Caracalla didn't survive that long after murdering his brother) but there were strategical changes as well- some of which we see in Severus's reign. Severus's change of strategic focus is interesting because it demonstrates the increasing foreignness of Rome from what it had been- and it demonstrates the way that the Empire evolved to meet new challenges- tougher enemies on the frontiers (especially in the East with the rise of Sassanid Persia) and the need to rule by consent in the provinces, and coopt local elites. Severus afterall probably spoke with a Carthaginian accent- (he might well have pronounced his own name Sheptimus Sheverus) Carthage three hundred years before his birth, had been Rome's great rival.
Severus's reign therefore is fascinating- Gibbon was right, though other reigns built towards it, it was a watershed. But 'decline' is the wrong image, rather we should think of a Severan transition- whereby under Marcus and Severus the Empire's nature changed and the old Rome slowly ebbed away to be replaced (after the shock of the mid-third century) with something very different, the empire of Diocletian and Constantine.