A Roman legion was the basic military unit of the ancient Roman army in the period of the late Roman Republic and the Roman Empire. It was roughly equivalent to the modern word division. In the plural, the legions, it may mean the entire Roman army.
A legion was about 5,000 men in several cohorts of heavy infantry (legionaries). It was usually accompanied by attached units of auxiliaries, who were not Roman citizens. They provided cavalry, ranged troops and skirmishers to complement the legion’s heavy infantry.
The size of a typical legion varied during the history of ancient Rome. It had a complement of 4,200 legionaries in the republican period of Rome. In the imperial period, the full complement was 5,500 men split into 10 cohorts of 480 men each. The first cohort was at double strength with 800 men. The remaining 220 were 120 cavalry plus technical staff.
Rome did not have a standing army until the reforms of Gaius Marius about 107 BC. Legions instead were created, used, and disbanded again. In the time of the early Roman Empire, there were usually about 25–35 standing legions plus their auxiliaries, with more raised as needed.
In the middle of the Republic, legions were composed of the following units:
- Equites (cavalry): The cavalry was originally the most prestigious unit, where wealthy young Roman men displayed their skill and prowess, laying the foundation for an eventual political career.
- Velites (light infantry): The velites were mainly poorer citizens who could not afford to equip themselves properly. Their primary function was to act as skirmishers – javelin-throwers who would engage the enemy early in order either to harass them or to cover the movement of troops behind them.
- Heavy infantry: This was the principal unit of the legion. The heavy infantry was composed of citizen legionaries that could afford the equipment composed of an iron helmet, shield, armour and pilum, a heavy javelin whose range was about 30 meters.
(Adapted from Roman legion Facts for Kids. Kiddle Encyclopedia. )