The Roman Army: The Force That Built an Empire

Colin Ricketts

3 mins

24 Jul 2018

Rome was almost a city built around an army. In the legend of the city’s founding father Romulus, one of his first acts is the creation of regiments called legions.

Romans weren’t any braver than their foes, and while their equipment was good, much of it was adapted from their enemies. If their military had one decisive edge it was its discipline, built on a rigid structure that meant every man knew his place and his duty, even in the chaos of hand-to-hand fighting.

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The origins of the Imperial Army

The foundations of the Imperial Army of 100 AD were laid by the first emperor, Augustus (ruled 30 BC – 14 AD).

He first reduced the army from its unsustainable civil war high of 50 legions to around 25.

Augustus wanted professional soldiers, not the armed civilians of the Republican era. Volunteers replaced conscripts, but with longer terms of service. To serve in a legion a man still had to be a Roman citizen.

He also reformed the chain of command, introducing the rank of legatus, a single, long-term commander for each legion. The traditional aristocratic commanders were reduced in status, and a praefectur castrorum (prefect of the camp) was appointed to oversee logistics.

An army of citizens and subjects

Auxilia on Trajans column

When the Roman legions marched, these elite citizen units were usually accompanied by an equal number of auxilia, as subject rather than citizen soldiers were called. The 25-year auxilia term was a route into citizenship that could be shortened by conspicuous bravery.

Auxilia were organised into cohorts of 500 men in infantry, cavalry and mixed formations. The men usually came from the same region or tribe, and for a while may have carried their own weapons. They were paid far less than the legionaries and less attention was paid to their organisation.

The anatomy of a legion

Roman centurion

Credit: Luc Viatour / Commons.

Many of the Marian Reforms of Gaius Marius in the 2nd century BC remained in place until the third century AD, including the legion structure defined by the man who saved Rome from invading German tribes.

A legion consisted of around 5,200 fighting men, sub-divided into a succession of smaller units.

Eight legionaries formed a contuberium, led by a decanus. They shared a tent, mule, grinding stone and cooking pot.

Ten of these units formed a centuria, led by a centurion and his chosen second-in-command, an optio.

Six centuria made up a cohort and the most senior centurion led the unit.

A first cohort was made up of five double-sized centuria. The most senior centurion in the legion led the unit as Primus Pilus. This was the legion’s elite unit.

Centuria or groups of them could be detached for a special purpose, when they became a vexillatio with their own commanding office.

By horse and by sea

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The Roman army of 100 AD was primarily an infantry force.

Officers would have ridden, and Augustus probably established a 120-strong mounted force with each legion, largely used for reconnaissance. Cavalry fighting was largely left to auxilia, whose mounted troops may have been paid more than standard legionaries, according to Arrian (86 – 160 AD), a soldier and writer.

No natural sea farers, the Romans were pushed into naval warfare, becoming proficient out of necessity and often with stolen ships.

Augustus considered the 700-ship navy he inherited from the civil wars his private property and sent slaves and freedmen to pull its oars and raise its sails. Further squadrons of ships were formed as the Empire expanded overseas and along great rivers like the Danube. Rome also relied on grain imported from Africa and needed to keep the Mediterranean free for trade.

Commanding a fleet as a praefecti was only open to Roman equestrians (one of the three ranks of the Roman nobility). Beneath them were navarchs in charge of squadrons of (probably) 10 ships, each captained by a trierarch. The ship’s crew were also led by a centurion and optio team – the Romans never really thought of their ships as more than floating platforms for infantry.