The Roman military was perhaps the most organized, efficient, and adaptable fighting force in the ancient world. See how complex and smartly structured the Roman military machine was.
The Roman army structure had two primary divisions. Roman citizens served in legions, and soldiers who were not citizens served in auxiliary cohorts. The auxiliary cohorts were made up of fighters from occupied lands in service of Rome. The Legions were structured in units called contubernium, with eight soldiers to a unit. Ten contubernium formed the larger century. Six centuries formed a cohort and ten cohorts, with a small cavalry unit, formed a legion. The auxiliaries were organized in a similar manner, though their largest unit was the cohort. Auxiliary soldiers who served for 25 years received a pension and all rights of a Roman citizen.