A scholarly study of the Roman army across the imperial period, tracing its development from the Augustan foundation of a standing professional force to the reforms and transformations associated with Constantine the Great. Drawing on archaeological evidence, inscriptions, literary sources, and military organization, the book examines how Rome recruited, trained, equipped, paid, and deployed its soldiers across a vast empire. It considers the changing roles of legions and auxiliary forces, the relationship between army and imperial power, and the pressures of frontier defense, civil war, and administrative change. Written for readers interested in ancient warfare and Roman institutions, it presents the army as both a military instrument and a central structure within the social, economic, and political life of the Roman Empire.