Structural history of the Roman military

The structural history of the Roman military concerns the major transformations in the organization and constitution of ancient Rome's armed forces, "the most effective and long-lived military institution known to history." At the highest level of structure, the forces were split into the Roman army and the Roman navy, although these two branches were less distinct than in many modern national defense forces. Within the top levels of both army and navy, structural changes occurred as a result of both positive military reform and organic structural evolution.

Source: Wikipedia — Structural history of the Roman military (CC BY-SA 4.0)