The typical Roman legion of the Late Republican era–at full strength–consisted of about 4800 troops organized into 10 cohorts. Each cohort consisted of six centuries of 80 to 100 men, depending upon the number of battlefield casualties, soldiers suffering from battle fatigue, and those who were ill. Each century was subdivided into contubernia, ten units of eight men.
Officers in the legions of the Late Republic consisted of
- The commander of the army was typically a Consul, a Proconsul, a Praetor or a Propraetor
- Legati, typically appointed by the Senate on advice from the governor of a province, could command one or more legions and/or handle various civil roles. However, in G. Julius Caesar's legions, 10 Legati plus a Quaestor were appointed by him alone rather than by the Senate.
- One or more Quaestores who superintended the business of an army; sort of a quartermaster general
- A Praefectus Equitum (officer of the horse), who commanded the cavalry
- A Praefectus Classis (naval officer)
- A Praefectus Fabrorum (officer of engineers)–who was not just an officer in charge of engineering and constructing projects, but rather was more of an aide-de-camp to the legionary commander.
- Each legion had six military Tribuni (Tribunes; literally "tribal officers") two at a time, rotated command of the legion daily so that no one officer became too powerful. The four off-duty tribunes acted as quartermasters, commissaries or aide-de-camps. All of them served mounted. These Tribunes were typically young men who hoped to eventually be admitted to the Senate
Centurio–commanded a Century
- In each cohort of the legion the highest ranking Centurions were two Pili (Pilus Prior and Pilus Posterior), two Princeps (Princeps Prior and Princeps Posterior), and two Hastati (Hastatus Prior and Hastatus Posterior) Centurions. During G. Julius Caesar's time, the Centurions of the first cohort ranked highest and the Centurions of the tenth cohort ranked lowest.
- The Centurio Primi Pili was the highest ranking of the Centurions; he was the Pilus Prior of the first cohort.
- Optio (rear-guard officer)–a Centurion's second in command
Other Legionary Staff
- Cohors Praetoria–consisting of lictors, secretaries, marshals, spies, servants and orderlies. (Note: Cohors Praetoria also refers to the commander's personal bodyguard)
- Comites Praetorii, volunteer aides, or if numerous a sort of gentlemen bodyguard
- Speculatores, each legion had at least ten, who acted as a vanguard and as flankers on the march. Generally sent out as scouts or spies.
- Antesignani–a few men from each cohort selected for service in delicate cases requiring skill and experience. They carried no baggage, and from them were often selected the Centurions.
- Fabri, or engineers, under the supervision of the Praefectus Fabrorum, repaired weapons, constructed bridges, siege mounds, towers, etc., . . . and planned and performed any other engineering work needed.
By Julius Caesar's time, the legionary cavalry (cavalry made up almost entirely of Roman equites) was a thing of the past. The bulk of the cavalry, by the Late Republic, was drawn instead from Rome's allies. Some of the more formidable cavalry were Numidians, Gauls, and Germans.
Cavalry served as exploratores (scouts) as well as point, flank, and rear-guard escorts for the army on the march. They also handled garrison duty, outpost duty, foraging, and the carrying of messages and dispatches. On the battlefield, the cavalry served on the flanks of the legion to protect the infantrymen and harried the enemy if they retreated.
A small cavalry detachment also served as the army commander's personal bodyguard. Those selected for this duty were often evocatii–veterans of past duty years who voluntarily remained in service.
Cavalry cohorts were known as alae (wings). Usually cavalry was divided into ten turmae (cavalry squadrons), consisting of approximately 30 men, one Decurio, and his three assistants called Optiones. It's thought that each turma rode in four ranks of eight; but of this there's no definitive proof.
Total numbers of cavalry during the Late Republic often varied depending on circumstances. For instance, at Palaepharsalus, Caesar had only about 1000 cavalry while Pompey's cavalry force numbered approximately 6700.
- The Praefectus Equitum or their own chief.
- Each turma was commanded by a Decurio, a Roman typically promoted from the ranks of the Legion. His duties were to: keep the troops in camp, train them, keep the keys to the gates (at permanent installations), ensure stable duties were performed effectively and efficiently; supervise the picket lines, make the rounds of the sentries at night, attend soldier's meals and sample the food to prevent the quartermasters from cheating, punish offenses, hear complaints and inspect sick quarters.
- Optio–a Decurion's second in command
During the Late Roman Republic, auxiliary contingents varied in size, and they weren't a regular arm of the Roman army. Auxilia were recruited from the provinces and sometimes organized into cohorts, called quingenaria, numbering 500 men amd divided like the legions into six centuries of approximately 80 men. They served as light infantry, archers (sagittarii), slingers (funditores), and swimmers whose job it was to guide horses across rivers.
Officers of the Auxilia:
Where auxiliary units were raised from a particular tribe, they were often commanded by leaders of those tribes. If the auxiliary unit was a more permanent one, it might be commanded by an ex-Centurion or ex-Tribune.
- Historians aren't clear as to the numbering system used by the legions during the Late Republic. Prior to this time four standing legions, numbered I through IV (1 through 4), were kept on call. Others were created as needed.
- The symbol of the Centurio's rank was the vitis (vine-staff), sort of a swagger stick, with which he meted out on-the-spot punishment when necessary.