Which was the largest military unit of the Roman army?

The Roman legion was the largest military unit of the Roman army, composed of 4,200 infantry and 300 equites (cavalry) in the period of the Roman Republic (509 BC–27 BC); and was composed of 5,200 infantry and 120 auxilia in the period of the Roman Empire (27 BC – AD 1453).

For most of the Roman Imperial period, the legions formed the Roman army's elite heavy infantry, recruited exclusively from Roman citizens, while the remainder of the army consisted of auxiliaries, who provided additional infantry and the vast majority of the Roman army's cavalry.

The military discipline of the legions was quite harsh. Regulations were strictly enforced, and a broad array of punishments could be inflicted upon a legionary who broke them. Many legionaries became devotees in the cult of the minor goddess Disciplina, whose virtues of frugality, severity and loyalty were central to their code of conduct and way of life.