The transition to a professional military had begun during the late Republic, and was one of the many profound shifts away from republicanism, under which an army of conscripts had exercised their responsibilities as citizens in defending the homeland in a campaign against a specific threat.
Overgangen til et professionelt militær var begyndt i slutningen af republikken, og var en af de mange dybe skift væk fra republikanismen, hvor man i stedet havde en hær af værnepligtige, som udøvede deres ansvar som borgere ved at forsvare hjemlandet i et felttog mod en specifik trussel.