The largest unit of the Roman army was the legion
The largest unit of the Ancient Roman Army was the legion. The Latin word "Legio", which ultimately means draft or levy, is where the word "legion" originates. There were initially just four Roman legions. These legions were numbered "I" through "IIII," with "I" rather than "IV" being inscribed for the fourth. The first legion was considered to be the most esteemed. Citizens made up the majority of the army and were assigned to a legion without their choice. By vote, any male from ages 16-46 was chosen and assigned to a legion.
The legion had grown from about 3,000 men in the Roman Republic to more than 5,200 men in the Roman Empire. By the 4th century AD, however, the legion was a much smaller unit of about 1,000 to 1,500 men. A corps is divided into groups of about 500 men. During this time, a typical Roman army had five or six legions. In the Roman army, the rank of the centurion was used. A rank in the Roman army called "Centuria" is comparable to a company in the modern era in terms of size and to a platoon in the modern era in terms of duty. The smallest part of the Roman legion was called "Contubernium". When on a campaign, its troops shared a tent or a barracks bunk room. Each legion was made up of 10 cohorts. In the initial cohort, there had five Centuria of 160 soldiers. The second through tenth cohorts had six Centuria with 80 males. Officers, cavalry, and archers are not among them.
The Romans adopted the Samnite manipular organization for their legions at this time, kept the levy system, and formed a permanent military alliance with all the other peninsular Italian republics. In order to participate in combined forces under Roman leadership, the latter was expected to provide nearly the same number of soldiers. In this period, the same number of allied alae (Roman non-citizen auxiliary), or forces about the size of legions, followed the legions on campaigns.
The Social War (91–88 BC) resulted in the granting of Roman citizenship to all Italians, the abolition of the ancient allied alae, and the incorporation of their members into the legions. The heart of legionary recruiting remained regular yearly conscription, but a rising number of recruits were volunteers who committed to 16-year terms as opposed to the maximum 6-year terms for conscripts. Due to the loss of the ala cavalry, the Roman and Italian legions' ability to provide cavalry cover was diminished by 75%. The legions, which were groups drawn from tribes inside Rome's distant empire and nearby allies tribes, were supplemented by the large-scale growth of native soldiers at this time. In the Roman provinces of Hispania, Gallia, and Thracia, large numbers of heavy infantry and cavalry, as well as archers from the Eastern Mediterranean, were recruited (mostly from Thrace, Anatolia, and Syria). These indigenous forces, however, did not become part of the legions. Instead, they maintained their own distinct leadership, structure, armor, and weaponry.