Usurpers are individuals or groups of individuals who obtain and maintain the power or rights of another by force and without legal authority. Usurpation was endemic during Roman imperial era, especially from the crisis of the third century onwards, when political instability became the rule.
The first dynasty of the Roman Empire, the Julio-Claudians (27 BC – 68 AD), justified the imperial throne by familial ties, namely with the connection (although only through adoption) with Augustus, the first emperor. Eventually conflicts within the Julio-Claudian family triggered a series of murders, which led to the demise of the line. Nero died with public enemy status, and following his suicide a short civil war began, known as the Year of the Four Emperors. The Flavian dynasty started with Vespasian only to end with the assassination of his second son Domitian. The 2nd century was a period of relative peace marked by the rule of the so-called Five Good Emperors, but the next century would be characterized by endemic political instability, one of the factors that eventually contributed to the fall of the Roman Empire in the West.
Commodus, the last emperor of the Antonine dynasty, remembered by contemporaneous chronicles as an unpopular ruler notorious for his extravagance and cruelty, was assassinated in 192. Without sons to be his heir, a struggle for power immediately broke out amongst the governors of the most important provinces. Pertinax was elevated to the purple and recognized by his peers, but following his murder by a restive Praetorian Guard, Septimius Severus decided to make his bid for power and usurped the throne. Although initially a usurper, Severus managed to remain in power for the next 18 years and died a natural death while campaigning in northern Britain. The death of Severus Alexander (the last emperor of the Severan dynasty) in 235 triggered what historians call the crisis of the third century. In this period, from 235 to the accession of Diocletian and the establishment of the Tetrarchy in 286, Rome saw 28 emperors of whom only two had a natural death (from the plague). However, there were also 38 usurpers who raised revolts across the Empire, a clear sign that the security of the frontiers was not the only problem within the Roman world. Usurpation attempts were a constant worry for the emperors in this period; it was a too common method of acceding the throne. Successful usurpers were usually either provincial governors, commanders of a large grouping of Roman legions, or prefects of the Praetorian Guard, which had control of Rome, where the Imperial palace still lay.
The danger of usurpation was greater following the death of an emperor, when his successor was not accepted by all provinces. Usually the legions acclaimed their own commander as emperor on news of the accession of a less popular man. The acclaimed emperor, usually a provincial governor, would then march to Italy or where the opponent was stationed, in order to contest for the purple. But legionaries disliked fighting against their brothers in arms, so battles between legions rarely transpired. Two main factors decided the success of a usurpation attempt: loyalty of the legionaries, heavily dependent on the amount of booty or monetary prizes promised on victory; and trust of the military abilities of the commander, upon which depended morale. Failure of either part to fulfill one or two of the criteria normally resulted in a mutiny and death at the hands of their own soldiers. Since the emperors had the status quo and political credibility behind them, the usurper had to be a charismatic man to avoid doubts in his ranks and an untimely death. Valerian, who defeated Aemilianus (himself a usurper) is an example of this kind. Other usurpers, like the emperor Philip the Arab, ascended the throne by a planned murder directed at an established sovereign (in his case Gordian III).
However successful, the usurpation procedure always left the new emperor in a somewhat fragile political position, since the throne had been attained by violent means. The danger of another usurper was always present and the first measures taken were inevitably to put trusted men into important commands. Frequently, the emperor embellished his ancestry and his early life in order to enhance his credibility or the right to the throne. Mentions of obscure genealogical relations with previous popular emperors were common, and certainly confused historians. But most of all the usurper manoeuvred to keep his legions happy, since he owed his power to their continued loyalty.
The usurpation mania of the 3rd century had profound effects in the bureaucratic and military organization of the Empire. Fear of potential rivals was to be the main driving force for the evolution of the Roman world from the early to the late Empire.
One of the most striking changes was the division and multiplication of the Roman provinces. Provinces were ruled by a governor, either a proconsul, propraetor or procurator, and were ascribed a certain number of legions, according to the degree of pacification they required. This meant that the governors of, for instance, Moesia or Pannonia in the Danubian border had huge military contingents on their hands. The greater the number of legions a provincial governor had, the greater the temptation to make a bid to the throne. And indeed, the majority of usurpation attempts came from the Asian province of Syria, and the Rhine and Danube provinces, frontier provinces with large military presence. Thus, provinces were slowly divided into smaller units to avoid concentration of power and military capacity in the hands of one man. Syria is a perfect example: a single province in AD 14, in the middle of the 3rd century it was divided in four different administrative regions: Tres Daciae, Cappadocia, Syria Coele and Syria Palestina. In a similar way, Moesia and Pannonia were divided in Superior and Inferior (Upper and Lower) halves; Dardania was later separated from Moesia, and Pannonia was further divided into Prima, Valeria, Savia and Secunda.
As the fear of civil war increased, the emperor felt the need of legions permanently in his reach, to be deployed against possible internal threats. This caused the geographic division of the army into limitanei legions, which remained in the borders, and comitatenses, which were stationed in strategic points within the Empire. Legio II Parthica, which was garrisoned in the Alban mountains outside of Rome from the time of Septimius Severus, was among the first comitatenses created.
Men had to be removed from the frontier garrisons to create these internal legions. A smaller number of border legions meant less secure borders and eventually raids from the Germanic and Gothic tribes against the Rhine and the Danube became more frequent. In the East, the Persian Empire grew bolder in their attacks on the Roman communities. Moreover, in a time when individual initiative was a common way to assume the imperial purple, giving important commands to competent generals was asking for trouble. Jealousy and fear often prevented the presence of the right man to deal with a specific threat, and, in consequence, marginal provinces were often raided, sacked or conquered.
The only usurpers whose early life and specific circumstances of rebellion are known with reasonable certainty are the ones who later became emperors. The unsuccessful usurpation attempts inevitably ended with the rebel's execution, murder or suicide and subsequent erasure of his life from all records. This often causes confusion in the contemporaneous sources which are contradictory in the details of a certain rebellion. For instance the usurper Uranius is placed by some in the reign of Elagabalus and by others in the time of Gallienus.
Every new emperor, either legal or illegal, marked the beginning of his rule by minting new coins, both for the prestige of declaring oneself as Augustus and to pay the loyal soldiers their share. Thus coinage is often the only evidence of a determined usurpation. But the number of coin types with the effigy of a usurper might not be equal to the total number of usurpations. The presence of minting facilities certainly allowed short term usurpers to release their coinage, but on the other hand, a man capable of sustaining a rebellion for a couple of months in a remote area might fail to produce his own coins due to the lack of access to the instruments of minting technology.
Later assessment of usurpation events demonstrated that some are questionable or even fictitious. Gallienus was the emperor who suffered greatest number of usurpations, with a record of fourteen attempts (excluding the Gallic Empire secession) in fifteen years of rule. However, three of these are clear fabrications, either contemporaneous to show the invincibility of the emperor or added by later writers to embellish their own prose.