The army was an institution of central importance throughout Roman imperial history. Its military effectiveness was obviously critical to the security of the empire, but as the empire's largest employer and the biggest single item on the imperial budget, it was also an organization whose impact on the economy was wide-ranging, while the ever-present danger of its being turned against a reigning emperor gave it a political dimension as well. In the late Roman period, all these issues assumed heightened significance. The more precarious strategic circumstances of the empire consequent upon the rise of Sasanian Persia to the east and the emergence of the Frankish, Alamannic and Gothic confederacies in the north meant that the army's military role was even more vital. Whatever the precise magnitude of the increase, the enlarged size of the fourth-century army had serious ramifications for the empire's manpower and economic resources. And the alarming frequency of military revolts during the mid third century must have left fourth-century emperors even more conscious of their vulnerability from this direction.

Two sources, very different in character, are especially valuable for late Roman military matters and deserve individual comment at the outset. The administrative document known as the Notitia Dignitatum enables us to see something of the formal organization of the empire's military forces – the army on paper, as it were – while the History of Ammianus Marcellinus allows us to observe the army in action, not only in its specifically military capacity, but also in its wider political and social context.