What is a Roman Legion? - Ranks, Size & Organization

Instructor: Christopher Sailus

Chris has an M.A. in history and taught university and high school history.

In this lesson we explore the history of the Roman Legion, its makeup, and how its organization and structure changed over the course of the Roman Empire.

The Roman Legion

Organization can be a pretty helpful thing. Having your desk organized, for example, can help you find things easier when you need them at work or school. If you're unlucky enough to ever by audited by the IRS, then having your financial records well-organized is extremely helpful.

In the army of ancient Rome, good organization was a matter of life and death. The Roman army was organized into several legions, and in this lesson we will explore the size and organization of the Roman legion and how it changed over time.

Pre-Marian Legion

The Roman Empire was arguably the largest and longest-sustaining empire in European history. That feat required a great army that was both formidable but also flexible to change. Indeed, the Roman legion changed over its history to meet the needs of the Empire. In the early, pre-empire days, the Roman Army fought largely as its Greek neighbors did: through the phalanx. The Greek phalanx was composed of spearmen who moved in unison as a fighting unit.

But the phalanx was most useful on large, open fields, and the undulating hills and valleys of the Italian peninsula required a fighting force that was more flexible. The Roman legion developed around the 4th century B.C. out of this need. The early legion was composed of roughly 5,000 troops that were organized into different classes of troops. The first class and the front line was the hastati. The hastati were relatively green troops that were equipped with one or two spears to throw when the enemy was near, and swords to fight in close combat.

In a second row, behind this group was the principes. The principes were more heavily armored and better equipped than the hastati, and they were the more experienced warriors. If the hastati took heavy losses--or worse, their lines broke--the principes would advance to engage the enemy, giving the hastati time to retreat and regroup.

Behind the principes were the triarii. The triarii formed the backbone of the Roman legion. A group of spearmen, the triarii were only called into service if the hastati and principes were being badly beaten. The triarii formed a close-knit line which served as a protective barrier for the rest of the retreating army.

Further troops, depending on the legion, included velites (light javelin-throwing troops who served as a first attack) and some cavalry units, though cavalry did not become prevalent until the later empire. Even these ranks of troops were highly organized. The basic unit of the Roman army was the maniple, which at first consisted of 60 men, but by the 2nd century B.C. this expanded to 120 men. Maniples were expected to act together, and each maniple was commanded individually by its own group of leaders during the heat of battle.

Marian Reforms and Later Legion

Near the end of the 2nd century B.C., the Roman legions underwent a series of reforms which helped them transform from a formidable fighting force to the best army of the ancient world. Under Gaius Marius, Roman general and statesman, the Roman legion became more professionalized. Marius opened the army to many more citizens than were previously eligible for service, lengthened service contracts and expanded training to create a class of highly professional soldiers. For their service, legionnaires were given promises of land in the Roman Empire and non-Romans were granted citizenship.

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