Overview

A Roman legion was the basic large military formation of ancient Rome, central to its expansion and security from the late Republic through the Roman Empire. The word "legion" could be used strictly for a tactical unit or more broadly, in the plural, to mean the armed forces of Rome as a whole. For general summaries of Roman military organization, see overviews of the Roman army and treatments of the late Republican period in which the legion evolved: late Republic studies. The legion continued as a dominant institution into the imperial era: imperial Roman military history.

Organization and composition

At its core a legion was a large body of heavy infantry trained and equipped for close combat, supported by non-citizen auxiliary units. Typical Republican and early Imperial arrangements differed somewhat, but a legion generally contained several cohorts, each made up of centuries (smaller companies). The heavy infantry were the legionaries; they were complemented by cavalry, slingers, archers and light troops provided by allied or auxiliary contingents. For a modern comparison in scale and function, the legion is often likened to a division in modern armed forces: comparative unit sizes.

Size and roles

The size of a legion varied over time. During the late Republic legions were commonly several thousand men, while in the early Empire a common organizational model was ten cohorts with the first cohort larger than the rest. Numbers quoted in sources range from around four thousand to five thousand or more when auxiliaries and specialists are included. Legions carried a broad set of functions beyond battlefield combat: they garrisoned frontiers, constructed forts and roads, undertook siege works and served as a mobile rapid-response force. Their attached auxiliaries supplied cavalry and skirmishing troops missing from the heavy infantry core: see further notes on auxiliaries and cavalry roles here and on legionary heavy infantry here.

History and development

Rome did not always maintain a permanently standing army. During the Republican era legions could be raised for particular campaigns and then disbanded. A major change came with the late Republican military reforms that professionalized legionary service and made legions into longer-term, standing units. These reforms are discussed in contemporary scholarship on military reform and professionalization; for background on the emergence of a standing Roman army see studies of Roman military reform. In the early imperial period the state typically kept multiple legions under arms, stationed across provinces to deter invasion, suppress revolt and maintain order.

Functions, equipment and legacy

Legions were intended to be largely self-sufficient. Besides fighting, legionaries built and maintained camps, roads and fortifications, and often served as a source of skilled labor in provincial projects. Each legion carried distinctive symbols and standards that fostered unit identity and morale; the loss or recovery of a standard could be politically and symbolically significant. When enlisted service ended, veterans were frequently settled in colonies, a practice that aided Romanization and the spread of Roman customs into the provinces. The plural term "legions" was sometimes used to refer to the whole Roman military establishment or to multiple such formations; for contextual use of the term in historical sources see usage of "legions".

Key features at a glance

For concise introductions and accessible overviews on specific aspects of legionary life, recruitment and operations, consult specialist treatments and museum summaries indexed under the links above. The Roman legion remains a key subject for understanding Roman military effectiveness, provincial administration and the cultural impact of Roman rule.