What Was the Roman Legion?

Feared and respected, the Roman legion was the backbone of the Roman army in both the Republican and the Imperial period. For centuries, the legionaries kept Rome safe, pushing its boundaries further. Highly disciplined, well-trained, -and organized, the Roman legion represented a pinnacle of military might in the ancient world. No wonder one of the most enduring images of ancient Rome is that of the legionary. But why was the Roman legion so successful? Was it its legendary discipline, leadership, equipment, or all of it? Read more to discover more on the Roman legion, the iconic unit that had no match among the militaries of the ancient world. 

The Roman legion was the core of the Roman army – the most powerful war machine in the ancient world. It is widely believed that the legionaries had no match among the opposing armies. Its soldiers were the embodiment of Rome’s military prowess and might. But was the Roman legion truly invincible? Not really. The legion caused fear in the opposing ranks and won many battles. But the legion also suffered horrendous defeats. The most notable ones are the catastrophe at Cannae in 216 BC, Carrhae in 53 BC, and Teutoburg Forest in 9 AD. In the latter, three legions were annihilated, never to be restored. However, the defeats on such a scale were uncommon, a fact that only further enhanced the legions’ reputation.

When Was the First Roman Legion Raised?

The legion served as the backbone of the Roman military machine from the times of the Republic. The first notable mention of the Roman legion comes from the writings of the Greek historian in Roman employ – Polybius – who described the legions of the mid-second century BC. That was the period of the Punic Wars when Rome fought for supremacy against its rival Carthage. During that period, the Roman legion was not a professional army. Instead, the legionaries were the citizen soldiers led by consuls. The Republican legion was an army raised for war and was disbanded after a single campaigning year.

How Was the Republican Legion Organized?

The more experienced principes (heavier infantry carrying large shields) stood in the middle of the line. Behind them was the elite veteran force – the triarii (heavily armed spearmen). At each flank of the main battle line was the cavalry, formed by the wealthy Roman aristocrats (the equites). According to Polybius, one legion numbered around 4500 men. In times of great need, such as Hannibal’s invasion of Italy, those numbers could be boosted up to 5000.

By the late second century, Rome obliterated Carthage and conquered Greece, setting its eye on the Hellenistic East. However, while initially successful, the citizen soldiers could not provide the long-term military force required for overseas expansion. Enter Gaius Marius. Realizing the limits of the obsolete military system, this Roman statesman and general pushed for sweeping military reform.

The citizen militia was replaced by professional soldiers whose sole purpose was to fight for Rome, both against the Roman enemies and in the civil wars, which became increasingly frequent. The result was a rapid increase in military manpower, as the army guaranteed a pension and provided the soldiers with a constant supply of food, pay, and medical care. The Marian reform turned the Roman legion into a powerful and professional standing army. But it also made the reformed legion the threat to the Roman Republic.

What Happened to the Roman Legion Under Augustus?

The professional legion contributed to the downfall of the Republic and the birth of the Roman Empire. Unlike the citizen militia, the professional soldiers had no interest in preserving the Republic. Thus, when Julius Caesar made his fateful decision and crossed the Rubicon, his troops blindly followed. Realizing the danger that a victorious general could pose to his position, the first emperor – Augustus – assumed complete control of the Roman military.

In addition, Augustus transferred all the legions to the frontiers, far from the center of power, limiting the potential usurpation. The only military force that remained in Rome was Augustus’ own creation – the Praetorian Guard. However, the Praetorians exploited their proximity to the throne, and following the assassination of emperor Caligula, they gradually turned into the kingmakers.

The basic unit of the Roman Imperial legion was the cohort, totalling 480 men. A cohort was comprised of six centuria, each led by a centurion. Interestingly, a century consisted not of 100 but of 80 men. The centuria were divided into contubernia. The contubernium (Latin for “tenting-together”) was the basic unit of the legion in the Roman Empire. It was a group of eight soldiers who lived in one tent and did everything together – from sharing the same cooking pot to fighting side by side in battle. The “band of brothers” created an unbreakable bond through 24 years of military service, remaining close friends after leaving the army and becoming veterans. 

How Big Was the Roman Legion?

The Roman legion numbers varied through time. In the Republican period, each legion consisted of around 4500 men. After the Marian reforms and during the early Empire, the legion size increased to approximately 5200 men, rising to 6000 on special occasions. Then it sharply dropped, following the reforms of Diocletian and Constantine the Great, to as few as 1000 men per legion. That does not mean that the legion became weaker. The late Roman army continued to fight and win many battles. And to adapt to the new threats, the cavalry’s numbers and importance increased. The flexibility of the Roman legion meant that even in the most critical periods, the unit could still function effectively. 

After all, when led by skilled generals like Belisarius, a small but well-trained military force could defeat a much larger opponent.