Over the course of its long history, the Roman Army underwent multiple reorganizations, usually in the aftermath of crushing defeats, and changed from a volunteer force composed entirely of property-owning citizens to a professional standing army. During the late Imperial period, the Roman Army gradually filled with barbarian mercenaries, until few differences existed between the "Roman" force and the enemies it met in battle.
In its classical form, that of the early Imperial period, the Roman Army was organized around large subdivisions called legions. The whole army consisted of around 22 legions, and each legion was comprised of around 5,000 men. Each legion was responsible for its own recruitment, provender and operations, though large operations often involved several legions working in concert. Each legion was subdivided into cohorts of around 480 men, and each cohort was likewise split into four or five centuries that were led by an officer called a centurion. Centurions were the key links in the chain of command. They wore distinctive armor, drew considerable pay and carried a staff, with which they were empowered to beat their subordinates. Beneath the centurion were Roman citizens who served as heavy infantry, known as legionaries, and foreign auxiliaries who worked as laborers, skirmishers and cavalry scouts.