According to Vegetius what led to the changes in Roman military armor and training techniques?

According to Vegetius, changes in Roman military armor and training techniques were primarily driven by the need to adapt to evolving warfare and threats from various enemies. He argued that earlier tactics and equipment became less effective due to changes in the nature of combat, emphasizing the importance of discipline, rigorous training, and the adoption of more effective armor. Vegetius believed that the decline of the Roman military's effectiveness was linked to a lack of proper training and the neglect of traditional military values. Ultimately, he advocated for a return to the practices that had once made the Roman legions successful.