As we read history, there are several cases since the mid 4th century that Hellenic military equipment, in general, was getting gradually lighter. In 399 BC, a hoplite would wear a muscled bronze cuirass, a Corinthian helmet, an Aspis, a pair of greaves and a spear with a reserve xyphos making him an effective heavy infantry soldier. That equipment weighed about 30 Kg, yet there seemed to be no significant problem with that for centuries.
But from 370 on, there were innovations: the massive use of peltasts, the introduction of the pelte and the thureos, both lighter than the Aspis, the abandon of heavy plate for lighter armour such as linothrax, and the disuse of the traditional Corinthian helmet. Some later Diadochoi elites tried to emulate the old times by restoring the old equipment in elite infantry such as the hypaspistai, and etc... but never to the same degree and scale as before.
Is there any valid and clear reason of why such trends changed the Greek army?
Bookmarks