The Roman Army in 117 CE
28 legions, hundreds of auxiliary units, and the household troops guarding Rome itself.
In 117 CE the Roman army stood at roughly 380,000 men under arms: 28 legions of Roman citizens, each about 5,500 strong, plus a roughly equal number again in auxiliary cohorts and cavalry alae recruited from the provinces. Trajan had just raised a new legion, II Traiana Fortis, for the Parthian campaign that was collapsing as this map's snapshot date arrived — the first legion added to the standing establishment in decades.
Every legion had a fixed home base, a castra legionis built to hold roughly 5,000 men behind earth-and-timber or stone walls, and most of those bases sat directly on a frontier: the Rhine, the Danube, the edge of the Syrian desert, the hills of northern Britain. A legion's name carried its own history — Legio X Fretensis ("of the strait") for the crossing that took it into the war against Judaea, Legio III Cyrenaica for a province it had garrisoned since Augustus.
Rome itself was garrisoned separately. The Praetorian Guard — about 10,000 men in ten cohorts, barracked at the Castra Praetoria on the city's northeast edge — protected the emperor's person and, more than once in the previous century, decided who the emperor would be. Smaller forces handled everyday order: the urban cohorts policed the city itself, and the vigiles, organized like a fire brigade crossed with a night watch, covered Rome's fourteen regions from guardhouses spread across the city.
The map below groups the 28 legionary fortresses by province, each one linked to its own place card with the fortress's Latin name and modern location, plus Rome's household troops — the Praetorian barracks, the urban cohorts, and the equites singulares Augusti, the emperor's mounted bodyguard.
Sources
- Emil Ritterling, "Legio" (Realencyclopädie der classischen Altertumswissenschaft, 1924-25)
- Yann Le Bohec, The Imperial Roman Army
- Public data: app/legions.ts, public/data/politics.geojson
- CAH XI, The High Empire, 70–192 CE (Cambridge Ancient History, 2nd ed.)
- Fergus Millar, The Roman Near East, 31 BC–AD 337
- Barrington Atlas of the Greek and Roman World
More to read
Roman Maps shows the empire frozen at 11 August 117 CE, the day Trajan died — its moment of greatest territorial extent. Explore the full map →