After the defeat and captivity of Valerian, the legions in the different provinces, while they agreed in scorning the feeble rule of Gallienus, could by no means unite their suffrages in favour of any one aspirant to the purple; but each army hastened to bestow the title of Augustus upon its favourite general. Hence arose within the short space of eight years (A. D. 260-267) no less than nineteen usurpers in the various dependencies of Rome, whose contests threatened speedily to produce the complete dissolution of the empire. The biographies of these adventurers, most of whom were of very humble origin, have been compiled by Trebellius Pollio, who has collected the whole under the fanciful designation of the Thirty Tyrants. But the analogy thus indicated will not bear examination. No parallel can be established between those pretenders who sprung up suddenly in diverse quarters of the world, without concert or sympathy, each struggling to obtain supreme dominion for himself, and that cabal which united under Critias and Theramenes with the common purpose of crushing the liberties of Athens. Nor does even the number correspond, for the Augustan historian is obliged to press in women and children and many doubtful names, in order to complete his tale. Of the whole nineteen, one only, Odenathus the Palmyrene, in gratitude for his successful valour against Sapor, was recognised by Gallienus as a colleague. It has been remarked, that not one lived in peace or died a natural death.
Among the last of the number was Aureolus, a Dacian by birth, by occupation originally a shepherd. His merits as a soldier were discovered by Valerian, who gave him high military rank; and he subsequently did good service in the wars waged against Ingenuus, Macrianus, and Postumus. He was at length induced to revolt, was proclaimed emperor by the legions of Illyria in the year 267, and made himself master of Northern Italy. Gallienus, having been recalled by this alarm from a campaign against the Goths, encountered and defeated his rebellious general, and shut him up in Milan; but, while prosecuting the siege with vigour, was assassinated. This catastrophe, however, did not long delay the fate of the usurper, who was the nearest enemy and consequently the first object of attack to his rival, the new emperor Claudius. Their pretensions were decided by a battle fought between Milan and Bergamo, in which Aureolus was slain; and the modern town of Pontirolo is said to represent under a corrupt form the name of the bridge (Pons Aureoli) thrown over the Adda at the spot where the victory was won. The records preserved of this period are full of confusion and contradiction.