The change in breastplates and helmets contribute to decline of the Roman Empire by weakening the the military. It weaken them because the removal of it promote laziness and negligence of training and the such. It also removes the protection against ranged weapons which caused many casualties.
The light infantry, especially at the beginning of the republic, fought without armor. If they had any on their tunic, it might have consisted of some hard leather breastplate. This was done both to allow these troops to move faster and for purely economic reasons. During much of the Republican period, the armor depended on the type the soldier could pay. And the first rows used a metal plate, usually of bronze, that covered the heart and part of the chest, held with leather strips. Those with more purchasing power used the mail coat. And the richest, expensive anatomical bronze armor, in the Greek style. Over the years, these anatomical armor remained in use by high-ranking officers.
