This is how the Roman Empire structured its military
The Roman Empire had one of the best militaries of ancient times. It steamrolled over the Carthaginians, Greeks, Egyptians, Gauls, more than held their own against many other forces for centuries.
So, how did the world's most powerful government organize its deadly legions? Well, it started with a group of eight men known as a contubernium – which was a little smaller than a typical infantry squad (usually nine personnel). Ten contuberniums, plus the command staff, formed a century of 85 men.
Six centuries – about 540 men – made up a cohort. This unit was roughly the size of a present-day infantry battalion. Ten cohorts, plus attachments including a force of cavalry, made a legion of about 6,000 men, roughly the size of a brigade.
The legions were the largest force in the Roman military. Only citizens could serve in the legions but the Roman military also had cohorts of auxiliaries which allowed non-citzens to serve in the Roman army.
These auxiliaries were usually slingers, archers, and additional cavalry. Many Roman citizens, who had to provide their own equipment, served as infantry.
After 25 years of service in the Roman army, legionnaires and auxiliaries could look forward to a generous retirement. Those who weren't citizens gained Roman citizenship and all that meant, plus a plot of land and a generous retirement bonus.