The Roman army was a highly organized and efficient military force, and its infantry units were the backbone of its strength. Here are some key Roman infantry units:

1. Legionaries

• Description: Legionaries were the heavy infantry of the Roman legions, composed of Roman citizens.
• Equipment: They were equipped with a gladius (short sword), pilum (javelin), scutum (large rectangular shield), and wore lorica segmentata (segmented armor) or chainmail.
• Structure: A legion was typically composed of around 4,800 to 5,200 men, divided into ten cohorts, each cohort containing six centuries of about 80 men each.

2. Auxiliaries (Auxilia)

• Description: Auxilia were non-citizen troops who supported the legions. They often provided specialized skills such as archery, cavalry, or light infantry.
• Equipment: Their equipment varied based on their role but was generally lighter than that of the legionaries.

3. Hastati

• Description: In the early Republic, Hastati were young, less experienced soldiers forming the first line of the triplex acies (three-line battle formation).
• Equipment: They carried a gladius, pilum, and a scutum and wore lighter armor than later legionaries.

4. Principes

• Description: More experienced soldiers than the Hastati, they formed the second line in the triplex acies.
• Equipment: Similar to the Hastati but generally better armored.

5. Triarii

• Description: The most experienced and heavily armored soldiers, forming the third line of the triplex acies.
• Equipment: They carried a hasta (spear) instead of a pilum and were used as a reserve force during battles.

6. Velites

• Description: Light infantry skirmishers used in the early Republic, composed of younger and poorer citizens.
• Equipment: They were lightly armored and carried a short sword, several javelins, and a small round shield.

7. Evocati

• Description: Veteran soldiers who had completed their term of service but voluntarily re-enlisted.
• Role: They often served as elite troops or in leadership roles due to their experience.

8. Cohortes Urbanae

• Description: Urban cohorts who acted as a police force within Rome and could serve as a military unit if necessary.
• Role: They maintained public order and were distinct from the legions and the Praetorian Guard.

9. Praetorian Guard

• Description: An elite unit that served as the personal bodyguard of the Roman emperors.
• Role: Besides protecting the emperor, they could also serve in battles and had significant political influence.

10. Numeri

• Description: Irregular units composed of non-Roman troops from various provinces or allied tribes.
• Equipment and Role: Varied widely based on their origin and specialization.

These infantry units together formed the core of the Roman military machine, enabling Rome to expand and maintain its vast empire over centuries.