Final answer:
Augustus defended the imperial borders through reforming the Roman army, establishing colonies, and integrating provincial subjects into Roman culture, providing stability rather than further expansion.
Step-by-step explanation:
To defend the imperial borders, Augustus used a combination of negotiation, buffer states, and local concentrations of troops. Instead of dramatically expanding Rome's frontiers, he focused on maintaining them through strategic fortifications and societal integration. Augustus reformed the Roman army, providing for a standing professional force with regular pay and retirement benefits. He also established Roman colonies for retired soldiers, spreading Roman culture and ensuring loyalty across the empire. Through such policies, Rome's boundaries were secure not just by force but also by fostering a sense of Roman identity among its subjects.