The Roman army developed continually from the third to the seventh centuries A.D., adapting to changes brought about by enemies beyond the imperial borders, as well as to changes in the structure of the Roman imperial state itself. Along with the change went much continuity and the army was always a standing force of long-service soldiers commanded by professional officers. This examination of the forces of the Roman Empire is divided into three parts: the structure of armies, the structure of regiments and the structure of individual careers.

Although the idea of an army often suggests rigid structures and placing square pegs into round holes, the forces of the Roman state were characterized by their structural flexibility and standard practices were often bypassed if necessary. Almost any imperial official could act as a military commander, regardless of whether he was of the ‘correct’ rank, or even if he was a soldier. Thus in 361 Iovius, who as quaestor sacri palatii usually dealt with imperial correspondence, commanded part of Julian’s army in the civil war against Constantius II and in 553 Bonus, who as quaestor exercitus was a senior supply officer on the lower Danube, was in charge of the garrison at Luca in Italy during the reconquest of Italy (Amm. Marc. 21.8.3; Agathias 1.19.1).