This book presents a new history of the leadership, organization, and disposition of the field armies of the east Roman empire between Julian (361-363) and Herakleios (610-641). To date, scholars studying this topic have privileged a poorly understood document, the Notitia dignitatum, and imposed it on the entire period from 395 to 630. This study, by contrast, gathers all of the available narrative, legal, papyrological, and epigraphic evidence to demonstrate empirically that the Notitia system emerged only in the 440s and that it was already mutating by the late fifth century before being fundamentally reformed during Justinian's wars of reconquest. This realization calls for a new, revised history of the eastern armies. Every facet of military policy must be reassessed, often with broad implications for the period. The volume provides a new military narrative for the period 361-630 and appendices revising the prosopography of high-ranking generals and arguing for a later Notitia.