Answer: The change of breastplates and helmets exposed the chest and head of foot soldiers of Rome. This allowed Goths to kill and injure them with arrows.
Explanation: Negligence and laziness of the soldiers made them abandon parade ground drills and also asked the Emperor to prevent them from wearing helmets and breastplates. In the battleground, the troops were exposed to archers due to lack of armour and the troops only thought about running instead of fighting. This in part leads to the decline of Rome.