legion, large unit of the Roman army. It came into prominence c.400 B.C. It originally consisted of 3,000 to 4,000 men drawn into eight ranks: the first six ranks, called hoplites, were heavily armed, while the last two, called velites, were only lightly armed. Marcus Furius Camillus is traditionally regarded as the great organizer of the legion. Under Camillus the hoplites were divided into three groups: the hastati (youngest men), the principes, and the triarii (oldest). Within the legion was the cohort, consisting of one maniple of each of the three groups plus 120 velites and a cavalry unit about 30 strong. A legion was composed of 10 cohorts and comprised about 5,000 men. In Caesar's time each legion had a commander who was responsible to the Senate, 6 tribunes, a legate, a prefect, and some 60 centurions. Training was hard, with much difficult drilling to prepare the men especially in shock tactics and for rapid marches. The standard weapons were the spear ( pilum ) and (after Scipio Africanus Major conquered Spain) the short thrusting sword ( gladius ). The characteristic emblems of the legions were eagles inscribed SPQR [ Senatus Populusque Romanus —the Senate and the people of Rome], and they carried the eagles in triumph over the far reaches of the empire for hundreds of years. Upon the legions rested to a large extent the glory of Rome. They were primarily heavy infantry and were vulnerable to quickly moving cavalry and archers (e.g., the defeat of Marcus Lucinius Crassus at Carrhae) and to guerrilla fighters (e.g., the famous defeat of Varus by the Germans). With the Germanic invasions the legion proved unable to match the barbarian horsemen, rendering it obsolete.
See G. Webster, The Roman Imperial Army of the First and Second Centuries (1969).
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