An army built for long wars
Rome’s conquests are often told as a series of campaigns, but they also depended on an institution: the army. The Roman legions did not win every battle, and Rome suffered serious defeats. Still, over centuries the army combined organisation, discipline and logistics in a way that gave it repeated advantages over rivals. This article looks at how that army worked, rather than at the order of its wars.
The legions of the Republic were originally citizen militias, raised for a campaign and sent home. Over time, and especially from the late second and early first centuries BCE, the army became increasingly professional, with soldiers who served for long periods. Ancient tradition credits the general Marius with reforms around 107 BCE that opened the ranks to poorer citizens, though historians think that change was gradual and not the work of one man.
The structure of a legion
A legion was a large unit of heavy infantry, typically numbering around five thousand men in the early empire on paper, though actual strength varied with losses and detachments. Its building blocks were:
- The contubernium. A group of eight men who shared a tent or room and mess.
- The century. Roughly eighty men, commanded by a centurion, the experienced professional officer who was the backbone of the army.
- The cohort. Six centuries, with ten cohorts making up a legion. In the early empire the first cohort was larger than the others.
Above these were senior officers, with a legate in overall command, supported by tribunes drawn from the upper classes. A legion also had a small cavalry contingent and specialist craftsmen. The army as a whole also included auxiliary troops, recruited from non-citizens in the provinces, who supplied cavalry, archers and light infantry. These soldiers later received citizenship on discharge.
The change from the older formation in three lines to cohorts is generally thought to have given more flexibility. Cohorts could be detached and sent to different tasks, or moved to support a threatened part of the line, something a rigid mass formation found harder. In contrast, the Greek hoplite phalanx relied on a dense block of men with long spears.
Equipment
A legionary typically carried a large curved shield, a short sword for thrusting, a heavy javelin called a pilum and a dagger. The pilum was designed to be thrown before contact and to bend or become hard to remove from a shield. Body protection varied by period, from mail to segmented plate armour, which is discussed in the study of ancient body armour. The standard weapons were not exotic, and the production of iron kit in large quantity reflected the ironworking spread of earlier centuries. Ancient weaponry in general was less decisive than the way men were trained to use it together.
Training and discipline
Recruits were drilled in marching, weapon handling and formation movement. Much of what we know about methods comes from later writers, including a late Roman author who wrote about military theory long after the early empire, so details need care. A frequently cited standard is that soldiers practised marching a set distance in a few hours while carrying equipment, but the exact figures are better treated as approximate.
Discipline was strict, and punishments could be severe. Rewards were also important: pay, a share of plunder, decorations and, on discharge, a bonus in money or land. The long service period, around twenty-five years in the empire, produced experienced veterans. A soldier’s identity centred on his century and legion, whose standards were treated with religious respect.
Medical support also developed over time, and the role of the Roman army surgeons shows that care for the wounded was part of the system.
Fortified camps
One of the most distinctive habits was the construction of a fortified camp at the end of a day’s march. Soldiers dug a ditch, raised an earth bank topped with stakes, and laid out tents on a standard plan. A Jewish historian who observed Roman armies in the first century CE described this practice with admiration. The result was that a Roman army was never easily surprised, and it always had a secure base to retreat to.
The remains of such sites, from small temporary enclosures to large forts, can be traced across Europe and the Near East. Their regular shapes are covered in the study of Roman marching camps. Standard layouts also made it easier for units to move in and settle quickly. The camp was part of a broader logistical effort, with supply trains, local purchases and roads built by the army itself. Those roads, in turn, shaped Roman road travel for civilians.
Siege warfare
Many conquests involved taking fortified places. Roman commanders used a combination of direct assault, mining, battering rams, towers and artillery that threw bolts and stones, together with the siege engines familiar from the Hellenistic world. When a town could not be taken quickly, the army would surround it with earthworks, as at Alesia in Gaul, cutting off supplies. At Masada in Judaea in the 70s CE, soldiers constructed a large ramp against the rock to reach the walls. These operations relied on engineering skill and a willingness to commit manpower for months.
Why the legions often won, and when they did not
Historians usually point to a combination of factors. Rome could replace losses from a large population and a network of allies. Its units were flexible, its officers were experienced, and its supply and engineering systems allowed it to sustain campaigns. Its political culture rewarded success in war. Defeat rarely led Rome to make peace on bad terms, and it often returned with fresh forces.
None of this made the army invincible. The disaster at Cannae in 216 BCE, the loss of an army at Carrhae in 53 BCE to Parthian mounted archers and the destruction of three legions in the Teutoburg Forest in 9 CE all show limits. Armies relied on good leadership, favourable ground and intelligence, and they could fail without them. Opponents also learned: Hannibal exploited the Roman battle line at Cannae, and Parthians used mobility and missiles to counter infantry. The army was a powerful tool, but it was one factor among several in the story of Roman expansion.