This work investigates the structure, organization, and societal role of the Roman army across various regions and eras. It examines the composition of legions, auxiliary units, and ethnic formations, detailing their ranks, career paths, and deployment across Europe, Asia, Egypt, and Africa. The book also touches upon the army's artistic and religious practices, concluding with an analysis of the later Roman army.

This foundational text in Roman military studies is recognized for its comprehensive scope and detailed examination of the Roman army's structure and function. Speidel's meticulous research, drawing on a wide array of historical evidence, provides an authoritative overview of the legions, auxiliaries, and the careers of soldiers. The book is considered an essential resource for understanding the military backbone of the Roman Empire, from its organization to its cultural impact.