A Roman legion was a large military unit of the ancient Roman Empire, typically consisting of about 4,000 to 6,000 soldiers. Each legion was made up of infantry, cavalry, and support troops, organized into smaller groups called cohorts and centuries. The legion was known for its discipline, training, and effective tactics, which contributed to the expansion and maintenance of Roman territories. Legions were commanded by a legatus, who was often a senator or an experienced military leader. Soldiers in a legion were called legionaries, and they served for a minimum of 25 years. The structure and organization of the legion allowed for flexibility in battle, making it a formidable force in ancient warfare.