 The mountainous geography of the region meant that the region was hard to subdue, but by 9 CE the Great Illyrian Revolt had been quelled and from then on the region would supply large numbers of non-citizen soldiers to the Roman Auxilia.The Romanization of these barbarian peoples eventually transformed them into the most valuable soldiers of the Late Roman Army, with a substantial portion of the officials and generals coming from a northern balkanic background, such as Illyria, Dalmatia, Pannonia and Moesia.One emperor, Decius, several usurpers and during the reign of Gallienus, who started the professionalization of the high command of the army, large numbers of soldiers achieved high rank within the army.