How big was a Roman legion?

After the reforms of the late 2nd and 1st centuries BC, a legion numbered roughly 4,800 to 5,000 heavy infantry, organised into ten cohorts, each cohort of several centuries, plus support troops. Earlier Republican legions were smaller and organised differently.

What weapons did a Roman legionary use?

The classic kit was the pilum, a heavy javelin thrown to disrupt the enemy line, the gladius, a short stabbing sword for close work, and the scutum, a large curved shield. Legionaries also dug, built roads and raised fortified camps every night.

What were the Marian reforms?

A set of changes associated with Gaius Marius around 107 BC that opened the army to the poor, by tradition supplied equipment at state expense (a practice partly older than Marius) and created a professional, long-service force organised in cohorts, replacing the older property-based militia.

Why was the Roman legion so effective?

Discipline, training, engineering and flexibility. Its cohorts could manoeuvre independently over any ground, it could fortify a camp anywhere, and it adapted to beat everything from the Greek phalanx to Hannibal's cavalry, though not before some catastrophic defeats.

How the Roman Legion Worked: the machine that built Rome

Rome did not conquer the ancient world with a secret weapon. It conquered with an organisation: a body of ordinary men drilled, equipped and led so well that it could lose battle after battle and still win the war. That organisation was the legion.

Key points

A legion was the basic unit of the Roman army, several thousand men strong, built from smaller units called cohorts and centuries.
Legionaries fought with the short sword (gladius), the heavy javelin (pilum) and a large shield (scutum).
Its strength was flexibility and discipline: it could change formation and keep fighting where a phalanx could not.
The reforms associated with Marius helped turn it into a long-service, increasingly professional army whose soldiers looked to their general; a true standing army came under Augustus.

Ranks and cohorts

By the late Republic a legion was around 4,800 to 5,000 heavy infantry¹, split into ten cohorts, each cohort made up of centuries led by a centurion. Earlier, in the manipular system, the line was drawn up in three staggered rows, the younger hastati in front, the seasoned principes behind², and the veteran triarii in reserve, arranged in a checkerboard that let fresh units filter forward through tired ones. Around this core rode cavalry and marched light skirmishers and, later, allied and auxiliary troops.

Relief of Roman legionaries with standards marching over a bridge of boats — Legionaries with their standards crossing the Danube on a pontoon bridge, carrying their kit on poles. Trajan's Column (AD 113), from the plates published by Conrad Cichorius in 1896. Public domain

The tools of the trade

The legionary's equipment was made for a particular way of fighting. He hurled the pilum, a heavy javelin designed to punch through shields and bend on impact¹, then closed with the gladius, a short sword built for stabbing at arm's length behind the big curved scutum shield. Just as important were the tools he carried: every night on campaign the legion dug in² and built a fortified camp.

Relief of Roman soldiers carrying blocks and building a walled camp — Legionaries building a fort while Trajan inspects the works, scenes from Trajan's Column (AD 113), from the plates published by Conrad Cichorius in 1896. Public domain

Not one weapon, but discipline, engineering and the will to adapt.

Reform and flexibility

Around 107 BC the reforms linked to Gaius Marius³ opened the ranks to the landless poor⁴, equipped them at the state's expense (as tradition holds, though the practice was partly older) and turned the legion into a professional, long-service force⁵. What made it unbeatable in the long run was flexibility: its cohorts could manoeuvre over any ground and adapt to any foe. The older manipular legion, with the same flexibility, had already beaten the rigid⁵ Greek phalanx at Cynoscephalae and Pydna, and Rome recovered even from disasters like Cannae, where Hannibal destroyed a far larger Roman army, before Julius Caesar later wielded the legions to conquer Gaul at Alesia. Brush up on the terms in our military history glossary.

Marble bust of an elderly Roman with a lined face looking upward — Marble bust in Munich traditionally called Gaius Marius, an Augustan copy of a portrait of a Roman of the 2nd century BC; the identification is not certain. Glyptothek, Munich. Public domain

Sources and notes

Numbered notes in the text point to the works below.

Adrian Goldsworthy, The Complete Roman Army (Thames & Hudson, 2003). ↩ a b
Polybius, Histories, Book 6, 19 to 42 (levy; hastati, principes and triarii; arms including the pilum; the marching camp). Loeb translation online at LacusCurtius: https://penelope.uchicago.edu/Thayer/E/Roman/Texts/Polybius/6*.html. ↩ a b
Sallust, The Jugurthine War (Bellum Iugurthinum), 86 (Marius enrols the capite censi in 107 BC). ↩
World History Encyclopedia, “Roman Army” ↩
Lawrence Keppie, The Making of the Roman Army: From Republic to Empire (Batsford, 1984). ↩ a b
Wikipedia, “Roman legion”

How the Roman Legion Worked