- Related Topics:
- military unit
- army
- maniple
- cohort
- formation
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- Humanities LibreTexts - The Army and Assimilation (Nov. 06, 2026)
legion
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What is a legion in military terms?
A legion was the largest permanent unit in the ancient Roman army, integral to Rome's military system for conquest and rule. The legion evolved from the Greek phalanx, becoming a flexible formation of well-coordinated units.
Key features of the legion:
* Structure Initially composed of maniples (small infantry units), it later transitioned to cohorts (larger units) of around 360 men each. A full legion consisted of about 10 cohorts.
* Tactics The legion utilized both shock and firepower, with soldiers wielding the pilum (javelin) and gladius (short sword).
* Flexibility Its flexible structure and command system allowed for quick adaptation on the battlefield, as demonstrated by Caesarās troops.
* Evolution Over time, the legion incorporated cavalry and artillery, becoming a combined-arms unit.2 Britannica SourcesCreated from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles. -
Which ancient civilization is most famous for using legions?
The ancient Roman civilization is most famous for its use of legions. The Roman legion was a military unit that helped the Roman Republic and Empire conquer and rule the ancient world.
The Roman legion evolved from the Greek phalanx formation because the Romans found it too difficult to use in the hills and valleys of central Italy. They created a new system using small infantry units called maniples. Each maniple had 120 men arranged in 12 files and 10 ranks. Ten cohorts comprised a legion, with a small cavalry force and supporting units, totaling about 10,000 men.
Two weapons that gave the legion its flexibility and force were the pilum, a 2-metre javelin, and the gladius, a 50-centimetre sword.
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How was a Roman legion organized?
A Roman legion was the largest permanent military organization in the Roman army. The Roman tactical system was based on maniples, small infantry units. Each maniple had 120 men in 12 files and 10 ranks. They were arranged in three lines, each with 10 maniples in a checkerboard pattern.
As Roman armies grew, the cohort, with about 360 men, replaced the maniple as the main tactical unit. A legion had 10 cohorts, with 4 in the first line and 3 each in the second and third lines. The 3,600 heavy infantry were supported by cavalry and light infantry, bringing the legion's strength to 6,000 men. Each legion also included a centurion, the commander of a centuria, the smallest unit of a Roman legion.
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What jobs did soldiers have within a legion?
Within a Roman legion, soldiers held various specialized roles. The centurion was a professional officer commanding a centuria, the smallest unit, typically of about 100 men. Legionaries were the backbone of the army, responsible for combat and construction tasks. There were also auxiliaries, supporting troops with specialized skills like slingers or cavalry. Some soldiers served as archers, slingers, or javelin men. As the empire evolved, the proportion of cavalry troops also increased. These various roles contributed to the legion's effectiveness in both offensive and defensive operations.
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How did legions help Rome build and control its empire?
The Roman legion was a flexible and formidable military unit that played a crucial role in Rome's expansion and control of its empire. The legion's effectiveness stemmed from its tactical system based on small, versatile infantry units called maniples, which were well-coordinated and integrated.
Legionaries were Roman citizens or provincials who became citizens upon enlistment or discharge, which helped Romanize the empire. They were long-service professionals who received pay, bonuses, and retirement benefits, incentivizing their loyalty and service. The army was used to defend the frontiers and enforce the Pax Romana, contributing to a single system and civilization in the Mediterranean world.
Beyond military might, legions constructed fortified camps, roads, and other infrastructure, facilitating trade, communication, and administration throughout the empire. These camps sometimes evolved into great European cities.
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How are modern military units different from ancient legions?
Modern military units differ significantly from ancient Roman legions in several key aspects.
| Category | Ancient Roman Legions | Modern Military Units |
| --- | --- | --- |
| Size & Structure | Composed of around 5,000-6,000 heavy infantry, supplemented by cavalry and light infantry.| Hierarchical structure including squads, platoons, companies, battalions, brigades, and divisions, with varying numbers of personnel.|
| Tactics & Weaponry | Relied on close-order combat using the gladius and pilum, with heavy infantry as the main force.| Employs combined arms tactics, integrating infantry, armor, artillery, and air support. Relies on firearms and advanced military technology.|
| Logistics & Support | Initially relied on self-containment and later on local supplies. Limited transportation technology, with a heavy reliance on human muscle for logistics.| Complex logistical systems for supply and transportation, using advanced technology and infrastructure.|
| Cavalry | Cavalry was a small supporting force.| Armored fighting vehicles replaced horses.|
| Flexibility | More flexible than the phalanx due to its articulation, command system, and standards.| Highly adaptable and specialized, with units designed for specific combat or support roles.|The Roman legion was a large, permanent military organization in ancient Rome, which conquered and ruled the ancient world. Modern military units have origins in the 16th-18th century, when professional armies reemerged in Europe.
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legion, a military organization, originally the largest permanent organization in the armies of ancient Rome. The term legion also denotes the military system by which imperial Rome conquered and ruled the ancient world.
The expanding early Roman Republic found the Greek phalanx formation too unwieldy for fragmented fighting in the hills and valleys of central Italy. Accordingly, the Romans evolved a new tactical system based on small and supple infantry units called maniples. Each maniple numbered 120 men in 12 files and 10 ranks. Maniples drew up for battle in three lines, each line made up of 10 maniples and the whole arranged in a checkerboard pattern. Separating each unit was an interval equivalent to a manipleās front of 18 m (60 feet), so that the maniples of the first line could fall back in defense into the intervals of the second line. Conversely, the second line could merge with the first to form a solid front 10 ranks deep and 360 m (1,200 feet) wide. In the third line, 10 maniples of light infantry were supplemented by smaller units of reserves. The three lines were 75 m (250 feet) apart, and from front to rear one maniple of each line formed a cohort of 420 men; this was the Roman equivalent of a battalion. Ten cohorts made up the heavy-infantry strength of a legion, but 20 cohorts were usually combined with a small cavalry force and other supporting units into a little self-supporting army of about 10,000 men.
Two infantry weapons gave the legion its famous flexibility and force; the pilum, a 2-metre (7-foot) javelin used for both throwing and thrusting; and the gladius, a 50-centimetre (20-inch) cut-and-thrust sword with a broad, heavy blade. For protecton each legionary had a metal helmet, cuirass, and convex shield. In battle, the first line of maniples attacked on the double, hurling javelins and then diving in with swords before the enemy had time to recover. Then came the maniples of the second line, and only a resolute foe could rally from the two successive shocks.
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As Roman armies of the late Republic and Empire became larger and more professional, the cohort, with an average field strength of 360 men, replaced the maniple as the chief tactical unit within legions. In the military operations of Lucius Cornelius Sulla and Julius Caesar, a legion was composed of 10 cohorts, with 4 cohorts in the first line and 3 each in the second and third lines. The 3,600 heavy infantry were supported by enough cavalry and light infantry to bring the legionās strength up to 6,000 men. Seven legions in three lines, comprising about 25,000 heavy infantry, occupied a mile and a half of front.
As Rome evolved from a conquering to a defending power, the cohort was increased to a field strength of 500ā600 men. These still depended on the shock tactics of pilum and gladius, but the 5,000ā6,000 heavy infantry in a legion were now combined with an equal number of supporting cavalry troops and light infantry made up of archers, slingers, and javelin men. In order to deal with mounted barbarian raiders, the proportion of cavalry rose from one-seventh to one-fourth. By the 4th century ad, with the empire defending its many fortified border outposts, as many as 10 catapults and 60 ballistae were assigned to each legion.
In modern times the term legion has been applied to a corps of foreign volunteers or mercenaries, such as the French provincial legions of Francis I and the second-line formations of Napoleon. āForeign legionā often signifies the irregular corps of foreign volunteers raised by states at war. The most famous of these is Franceās Foreign Legion (LĆ©gion ĆtrangĆØre); composed of foreign volunteers and commanded by French officers, it has served in various parts of the French colonial empire since its founding in 1831.