This chapter examines the military forces of the Roman Empire in three parts: the structure of armies, the structure of regiments, and the structure of individual careers. The first section of the chapter deals with the structure of the Roman army above the level of operational units. All late Roman armies were made up of individual regiments that had their own histories and traditions. The second section focuses on establishment strengths, though regimental units were under strength for much of their existence. The third part deals with the structures of units made up of individuals. It is sometimes argued that the late Roman army suffered from severe shortages of manpower and was thus forced to rely on non-Roman manpower. Soldiers were either conscripts or volunteers, but it is not possible to assess the relative importance of the two. Conscripts also came from annual levies of both free Romans and barbarians settled within the empire.

This chapter considers the military capacities and costs of different military forces. These capacities and costs, however, involved considerations rather more complex than, for example, the limited ability of arrows to pierce hoplite armour. The chapter covers the period from the lifting of the Dark Age (c. 750) to the end of the classical period (338). In 338 the Macedonian army of Philip II defeated a coalition of the most powerful Greek city-states, Athens, Thebes and Corinth, established Macedonian dominance over mainland Greece and put an end to hoplite dominance of land warfare. A brief description serves to sum up the treatment of military forces, since the Macedonian army in many ways represented the culmination of classical trends. The Macedonian army was powerful, not only because of the phalangite who replaced the hoplite as the mainstay of the infantry, but also because of the coordinated use of different types of military forces: cavalry of different types, peltasts, slingers and archers.