The Roman legion was not a fixed thing. It evolved over seven centuries from the early Republic’s tribal levies to the late Empire’s frontier garrison forces, changing in size, structure, equipment, and recruitment as the military demands on Rome changed. What remained constant was the underlying principle: an infantry force organized for sustained close-quarters combat, disciplined enough to function as a unit under conditions that destroyed individual cohesion, and administratively sophisticated enough to function as a self-sustaining organization in the field.