Roman Legion Organization

Timeline

1. c. -100–1Italian Peninsula · Italian PeninsulaIn the late Republic the legion transitioned from citizen militia to professional soldiers, forming an organization built on cohorts, centuries, and contubernia, with standardized equipment and training systems established.
2. c. 1–250Rhine and Danube frontiers · Roman Empire frontiersIn the early Empire legions garrisoned long-term along the Rhine, Danube, and Syrian frontiers, handling field operations, fortification, and security while absorbing large numbers of auxiliary troops and local recruits.
3. c. 250–500Western Roman Empire · Western Roman EmpireAfter the third century legions gradually became more mobile and cavalry-oriented, with changes in organization and recruitment; after the collapse of the West the traditional legion system gave way to barbarian war leaders and the new Eastern Roman military system.