A Roman legion was the main fighting unit of the Roman army, made up of about 5,000 heavy infantry who were Roman citizens. Each legion was split into cohorts and was based for years at a time in a permanent fortress.
Key takeaways
- A legion in the early empire held roughly 5,000 citizen soldiers, split into ten cohorts, and the first cohort was nearly double the size of the rest [1].
- Legions were backed by auxiliary troops who were not citizens, and for most of the empire these auxiliaries supplied most of the army's manpower [1].
- Our map has no legion or fort layer, but it does show the 43 provinces of AD 200 that the legions guarded.
- A legion sat in a fixed base, so a province with a legion in it usually felt very different from a peaceful interior province.
How a Roman legion worked
The legion was the heart of the Roman army. In the early empire a legion held about 5,000 heavy infantry, and the sources differ slightly on the exact figure [1]. The men were organised into ten cohorts. Nine of them were of standard size, around 480 men, while the first cohort was almost double strength at about 800 [1]. Historians note that the numbers on paper were often higher than the numbers actually present, so any single figure is a guide and not a count.
Legionaries were Roman citizens. Imperial legions recruited exclusively from citizens, which is why the army was such an important route into the Roman state for people who had gained citizenship [1]. A recruit signed on for a long term of service, and in return received pay and, at the end, usually a grant of land or money.
Alongside the legions fought the auxilia, units of non-citizen soldiers. Under Augustus the auxiliaries grew until they were about as numerous as the legionaries, and for most of the imperial period they supplied the army's cavalry and much of its extra infantry [1]. A non-citizen who was honourably discharged from the auxiliaries could gain citizenship [1].
Legions did not march about the empire all the time. Each one was assigned to a permanent base called a castra legionaria, a legionary fortress [1]. From there it could be sent out on campaign, or split up to build roads, bridges and walls. That is why legions are so closely tied to the frontier zones, which the homepage explains in why the borders are approximate.
An example from our data
The map does not show legions, forts or walls. What it does show is the ground they worked on. Take Britannia, which Rome annexed in AD 43 and whose outline covers 170,292 km². Its capital was Londinium, and the modern United Kingdom has 173 Roman places in the Pleiades gazetteer on our cities layer. A province like this was held by an army, but the army itself is not drawn.
To see the same idea at the empire's peak, open AD 117, when the empire reached 4.88 million km² across 41 provinces. Every one of those frontiers needed troops, and the legions were spread thinly along them.
The thing people get wrong
Films often show a "Roman legion" as a vast army of tens of thousands. A single legion was much smaller, about 5,000 men with its own camp, engineers and officers [1]. An army on campaign might gather several legions and many auxiliaries, but the legion itself was a unit with a fixed place in the army, not the whole army.
A second mistake is to think legionaries were mainly Italians fighting far from home. Over time, recruits came more and more from the provinces where the legions were based, though they still had to be citizens. Our Roman citizenship entry explains how provincials gained that status.
Legion vs auxilia
| Legion | Auxilia | |
|---|---|---|
| Who served | Roman citizens | Mostly non-citizens |
| Main role | Heavy infantry | Cavalry, light infantry, specialists |
| Reward on discharge | Land or money | Roman citizenship [1] |
Frequently asked questions
How many soldiers were in a Roman legion?
Sources give slightly different figures, but in the early empire a legion held about 5,000 men, in ten cohorts, with a larger first cohort. The paper strength was often higher than the real strength.
Could anyone join a Roman legion?
Imperial legions recruited from Roman citizens. Non-citizens could serve in the auxiliary units instead, and gain citizenship when they were honourably discharged.
Does the map show where the legions were?
No. The map shows provinces, cities, roads and borders, but no legions or forts.
Sources
- Roman legion, Wikipedia.
- Every figure from our own map is worked out from the datasets on the data sources page (checked against Ancient World Mapping Center outlines and Pleiades).