The power of Ancient Rome was built on a foundation of military expertise, honed and adapted over centuries of warfare with neighbouring powers that allowed them to conquer and control the entire Mediterranean basin and beyond for many centuries.
The Early Roman Army (c. 500-300 BC)
The Early Roman army was in use from the Regal Period up to the end of the early Republican period in c. 300 BC, when the ‘Polybian’ or manipular system was introduced.
Until c. 550 BC, the Roman army consisted of clan-based war-bands, which only came together in periods of war. King Servius Tullius instigated a universal levy of eligible adult male citizens, which was held at the start of each campaigning season and was disbanded at the end of the season. The levy would have consisted of 9,000 men, of whom 6,000 would be heavily armed infantry (probably Greek-style hoplites), 2,400 light-armed infantry, called velites, and 600 light cavalry, who would fight in the Greek-style phalanx. When the Republican period began in c. 500 BC, the levy remained the same but was now divided equally between the two consuls, each commanding one legion of 4,500 men.
The foedus Cassianum agreed with the Latins in c. 493 BC, which was a mutual non-aggression and defensive pact, among other things provided the Roman army with an equal number of Latin troops, whcih effectively double the size of the army to 18,000.
The Early Roman army participated in the following conflicts:
- The Etruscan Wars
- The First and Second Samnite War
- The Latin War
The Roman Army of the mid-Republic (c. 300-88 BC)
The mid-Republican army was in use from the end of the Second Samnite War c. 300 BC to the end of the Social War in 88 BC.
The central feature of the mid-Republican army was the manipular system that replaced the phalanx of the Early Roman army. The army was now drawn up in three lines (triplex acies) consisting of small units called maniples of 120 men, arrayed in chessboard fashion, giving much greater tactical strength and flexibility. This adaptation came about after a series of defeats in the rugged territory of the Samnites, which was not conducive to the phalanx formation. At this time, the number of legions grew from 2 to 4 and each legion was now supported by a force of non-citizens of roughly equal size, the ala, recruited from Rome’s Italian allies called socii.
The normal size of a legion in the mid-Republic was 4,200 infantry, of whom 3,000 were heavy infantry and 1,200 were velites plus 200–300 cavalry. There were 10 maniples in each of the three battle lines organised into hastati, principes and triarii. The maniples of the hastati and principes contained twice as many men (120) as those in the triarii (60). Membership of each line was determined by age-group: the hastati contained the younger men (up to 25 years old); the principes those in the 26–35 group; and the triarii the older men (36–46).
At this time, the Roman army still consisted of levies but after the Second Punic War, Rome acquired an overseas empire and so needed to maintain an army in the provinces. And so the levies were supplemented by a large number of volunteers, who were willing to serve for much longer than the legal six-year limit. These volunteers were mainly from the poorest social class, who did not own land and were attracted by the modest military pay and the prospect of a share of war booty.
In 107 BC, C. Marius implemented wide-sweeping changes to the mid-Republican army called the ‘Marian Reforms’. His intention was to create a more professional and permanent Roman army. These reforms revolutionized the Roman military machine, introducing the standardized legionary, the cohort unit and drastically altering the property and weaponry requirements for recruitment. The reforms also put the responsibility of supplying and managing an army in the hands of the general. Marius also granted citizenship and land to all Roman soldiers. The consequences of his reforms had a significant impact on the military supremacy of Rome as well as unintentionally contributing to the Late Republic’s social and political disruption.
The mid-Republican Roman army participated in the following conflicts:
- The Third Samnite War
- The Punic Wars
- The Pyrrhic War
- The Roman-Gallic Wars
- The Macedonian Wars
- The Achaean War
- The Servile Wars
- The Jugurthine War
- The Cimbrian War
- The Social War
The Roman Army of the Late Republic (88-30 BC)
The Late Republican army was in use from the end of the Social War in 88 BC until the establishment of the Imperial Roman army by Augustus in 30 BC.
After the end of the Social War in 88 BC, Roman citizenship was granted to all its Italian allies, or socii, south of the Po River, which meant that the alae were abolished and the socii were from now on recruited directly into the Roman legions. The Late Republican army saw itself thrust into the forefront of politics as increasingly the legions showed loyalty to individual commanders rather than the Roman state with the promise of booty and land. For this reason, the majority of fighting occured in a series of civil wars against the legions of another Roman commander vying for control of the Republic. This ended when Augustus, then Octavian, came out on top against Mark Anthony and created the Principate.
The Late Republican Roman army participated in the following conflicts:
- The Mithridatic Wars
- The Civil War of Marius and Sulla
- The Sertorian War
- The Third Servile War
- The Gallic Wars
- Crassus’ Failed Campaign in Parthia
- The Civil War of Julius Caesar
- The Liberators’ Civil War
- The Civil War of Octavian and Mark Anthony
The Imperial Roman Army (30 BC – 284 AD)
The Imperial Roman army was in use from its establishment by Augustus in 30 BC to until the reforms of Diocletian in 284 AD.
At the start of this period, Augustus replaced the Republican system of citizen conscription with a standing professional army of mainly volunteers, who served a standard 20-year term with five years a reservist. The conscription of citizens was only decreed in emergencies, such as during the Great Illyrian revolt of 6-9 AD. Under Augustus, there were 28 legions, consisting of c. 5,000 men each, totaling 140,000 heavy infantry overall. By Septimius Severus, this had increased to 33 legions of c. 5,500 men each, totaling 181,500 overall. The Roman legions were supported by the Auxilia, who were units of non-citizens from within the Empire. They served a minimum term of 25 years and would receive citizenship for themelsves and their families upon retirement. Under Augustus, the Auxilia consisted of about 250 cohorts and alae of c. 500 men, totaling 125,000 overall. By Septimius Severus, this had increased to around 400, totaling 200,000 men overall. The Auxilia contained infantry, cavalry and specialised units from across the Empire, such as Balearic slingers, Cretan archers and Numidian light cavalry.
The Impeiral Roman army participated in the following conflicts:
- Cantabrian Wars
- Campaigns in Germania
- Conquest of Britannia
- Roman-Parthian Wars
- Jewish-Roman Wars
- Year of the Four Emperors
- Dacian Wars
- Marcomannic Wars
- Septimius Severus’ Civil War
- Invasion of Caledonia
- Persian Wars
Reading Lists on the Roman Army
Digital Maps of the Ancient World 