Trajan Decius in the context of "Herennius Etruscus"

The Illyrian emperors (Latin: Illyriciani) were a group of Roman emperors during the Crisis of the Third Century who were of Illyrian origin and hailed from the region of Illyria (Latin: Illyricum, in the Western Balkans), and were raised chiefly from the ranks of the Roman army (whence they are ranked among the so-called "barracks emperors"). In the empire the Illyrian generals had established a powerful military caste.

In the 2nd and 3rd centuries, the Illyricum, which included the provinces on the Lower Danube (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.

The Battle of Beroe was a conflict near Stara Zagora, ancient Ulpia Augusta Traiana, between the Romans and Goths in 250. The Romans under Trajan Decius were pursuing the Goths, under Cniva, after their victory at the Battle of Nicopolis ad Istrum. A Goth ambush at Beroe led to heavy Roman losses and forced Decius to retreat across the Balkan Mountains to Moesia. This allowed Cniva to capture Philippopolis in Thrace.