The Legions: first organized military

A Roman legion had roughly 4,200 to 5,500 soldiers, depending on the period. In the late Republic and early Empire, it was about 4,800 to 5,300 legionaries in ten cohorts, with each cohort split into six centuries of about 80 men. Counts that include servants and other non-fighters could reach about 6,000.

A legionary served 16 years at first, and the term was later extended to 20 and then 25. Recruitment was mostly voluntary, discipline was harsh, and conditions were often tough. Pay was good, though, and those honorably discharged received a one-time payment and a parcel of land.

Each legionary carried a lot of their own gear. He wore a helmet and cuirass and fought with a shield, a spear called a pilum, and a short sword called a gladius, and he also carried tools and supplies on his back. Soldiers were nicknamed Marius' Mules because of the heavy loads they carried.

Gaius Marius turned the part-time citizen militia into a full-time professional army, a change dated to 107 BCE. He loosened enlistment rules so poorer citizens could join, standardized training and equipment, and made the cohort, not the older maniple, the main unit. The army became more flexible and offered soldiers a career.