During the Roman Empire, the makeup of the legion evolved significantly, shifting from a citizen-based military force to one increasingly composed of non-citizen recruits. This change was driven by the need for more soldiers to defend the expansive borders and maintain order within the empire. As a result, legions began to include provincials and mercenaries, which diversified the composition and skills of the troops. Additionally, the focus on professional soldiers over temporary conscripts marked a significant transformation in the Roman military structure.