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 maniple's 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.

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 legion's 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 500–600 men. These still depended on the shock tactics of pilum and gladius, but the 5,000–6,000 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. “Foreign legion” often signifies the irregular corps of foreign volunteers raised by states at war. The most famous of these is France's Foreign Legion (Foreign Legion, French) (Légion Étrangère); 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.