In the 2nd and 3rd centuries, the Illyricum and the other Danubian provinces (Dacia, Raetia, Pannonia, Moesia) held the largest concentration of Roman forces (12 legions, up to a third of the total army), and were a major recruiting ground. The advance of these low-born provincials was facilitated by a major shift in imperial policy from the time of Gallienus (253–268) on, when higher military appointments ceased to be exclusively filled by senators. Instead, professional soldiers of humble origin who had risen through the ranks to the post of primus pilus (which also entailed admission to the equestrian order) were placed as heads of the legions and filled the army's command structure.[citation needed]
Since Decius hailed from the senatorial background, the historical period of the Illyrian emperors proper begins with Claudius Gothicus in 268 and continues in 284 with the rise of Diocletian and the institution of the Tetrarchy.[3] This period was very important in the history of the Empire, since it represents the recovery from the Crisis of the Third Century, a long period of usurpations and military difficulties.
