Legion, the basic infantry unit of the Roman army. Legions were originally small groups of armed citizens sent to deal with threats to the city. Their size later grew to between 3,600 and 6,000 men. The legion was a strong, mobile force, capable of closing in on the enemy with surprising speed. Roman legions won for Rome its vast empire and maintained it for hundreds of years. After 300 a.d., the size of the legion was reduced and cavalry became more important than infantry.
The smallest unit in the legion was a century, composed of 60 to 100 men and led by a centurion. Two centuries formed a maniple, three maniples made a cohort, and 10 cohorts made up a legion. The commander of a legion was a tribune. The centurion of the leading unit in a formation carried the legion's standard, which was (after 100 B.C.) a silver eagle. Loss of a standard in battle was the deepest disgrace a legion could incur.
In ordinary battle array a legion was drawn up in three lines. When the first line began the battle, the second, 50 paces to the rear, moved up ready to replace the first line as soon as it fell or became weary. The third line, composed of older men and untried youths, was brought into action only in emergency.
Each soldier's equipment included his uniform, his weapons, and his pack. His uniform consisted of a tunic, cloak, and sandals. In battle he added to these a metal-reinforced leather shield. His weapons were a six-foot (1.8-m) javelin and a short two-edged sword. In his pack were his wheat allowance, cooking utensils, tent stakes, and an ax and spade. The pack weighed from 50 to 60 pounds (23-27 kg). During battle it was left under guard in the camp.
The first legionnaires were civil conscripts, but after 100 B.C. they were always professionals. By 150 A.D. more of them came from the provinces than from Italy. All legionnaires were Roman citizens and enjoyed better pay, conditions, and reputation than the auxiliary troops, who were not citizens. After 250 A.D. Rome found it increasingly difficult to pay the legionnaires. Many of them were settled on the frontiers, where they served as a peasant militia rather than as a standing army.