The primary components of the Roman military in the later Roman Republic and the early Roman Empire were the legions, large, independent, combined-arms infantry formations. Each legion consisted of around 5,000 soldiers, called legionaries. The legions were numbered and organized into fixed formations, with each legion under the command of a senior military tribune, or legatus. The legion was divided into 60 centuriae, each composed of 80 soldiers led by a centurion. The centurions reported to senior subordinates: the primipili, or chief centurions, and the tribunes. The legion also included specialist troops such as cavalry and artillery, as well as an administrative staff that handled recruitment, logistics, and supplies.
Auxiliaries
In addition to the legions, the Roman military also employed auxiliary units, known as auxilia. These were non-Roman troops recruited from various regions throughout the empire. The auxilia were organized into cohorts of 500-1,000 troops and alae of cavalry, with smaller subunits at the squadron, century, or turma level. The auxiliaries were under the command of officers with equestrian status, known as praefecti cohortis or praefecti equitum, and were supported by a staff of auxiliary centurions.
Praetorian Guard
The Praetorian Guard was an elite unit that protected the emperor and his family. It consisted of 9 cohorts, each with 500-1,000 men. The Praetorians were chosen from among the most experienced soldiers and received special training and privileges. They were under the direct authority of the emperor and played an important role in political affairs and imperial succession.
Cavalry
The Roman military also included cavalry units, known as equites or alae. The cavalry was mainly recruited from among the Roman upper classes and served as scouts, raiders, and shock troops. The cavalry units were organized into squadrons (turmae) of around 30-40 horsemen, commanded by decurions. The cavalry was under the overall command of a senior officer, such as a praefectus equitum or a legatus.
Artillery
The Roman military employed a variety of artillery pieces, including ballistae, onagers, and scorpions. These weapons were used to launch projectiles at enemy formations, fortifications, and ships. The artillery was organized into units of varying size, called armamentaria, and was operated by specialized artillerymen, known as optiones.
Officer Ranks
The Roman military had a hierarchical officer structure, with ranks based on experience, seniority, and military ability. The highest-ranking officers were the legati legionis, or legates, who commanded the legions. The tribunes served as the senior officers within the legions, with responsibility for discipline, training, and administration. The centurions were the junior officers who led the centuries and were responsible for tactical command and combat operations.
Overall Organization
The Roman military was divided into multiple armies and provinces. Each army was commanded by a general, who reported to the emperor or his designated military superior. The provinces were administered by governors who also served as military commanders within their respective regions. The overall command and control of the Roman military were centralized in the hands of the emperor, who served as the supreme commander and had ultimate authority over all military matters.