Commodus, the last emperor of the Nerva–Antonine dynasty, was remembered by contemporaneous chronicles as an unpopular ruler notorious for his extravagance and cruelty, and he was assassinated in 192.
Pertinax was elevated to the purple and recognized by his peers, but after 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 he was campaigning in northern Britain.
Successful usurpers were usually 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.
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.
Thus, provinces were slowly divided into smaller units to avoid concentration of power and military capacity in the hands of one man.
That 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.
Jealousy and fear often prevented the presence of the right man to deal with a specific threat, and so marginal provinces were often raided, sacked or conquered.
The unsuccessful usurpation attempts inevitably ended with the rebel's execution, murder or suicide and the subsequent erasure of his life from all records.
Every new emperor, either legal or illegal, marked the beginning of his rule by minting new coins, both to have the prestige of declaring oneself as Augustus and to pay the loyal soldiers their share.
Gallienus was the emperor who suffered greatest number of usurpations, with a record of 14 attempts (excluding the Gallic Empire secession) in 15 years of rule.