Gallic Wars
Political background
Caesar had initially four veteran legions under his direct command: Legio VII, Legio VIII, Legio IX Hispana, and Legio X. As he had been Governor of Hispania Ulterior in 61 BC and had campaigned successfully with them against the Lusitanians, Caesar knew personally most (perhaps even all) of these legions.[citation needed] Caesar also had the legal authority to levy additional legions and auxiliary units as he saw fit. The assignment of the provinces that comprise what we today know as Northern Italy was also helpful to his ambitions: the Po Valley and the adjoining regions had large numbers of Roman citizens, who could be enticed to sign up for legionary service.
Gallic Wars-4