It seems that there was a shift from the somewhat heavier, more expensive bronze cuirass to lighter, more cheaper forms of body armor such as the linothorax in the Greek city states. Was this just a way to cheaply equip mass amounts of troops? Did richer, more elite soldiers continue to use metal cuirasses? How widespread was the move to lighter forms of armor? Is it possible soldiers carried both a metal cuirass and a linothorax with them while on the move?
The argument for the lightening of the panoply was first made by J.K. Anderson,^1 mostly on the basis of artistic representations of hoplite armour. He noted that hoplites on vases of the late 6th century BC tended to wear full bronze gear, but those of the 5th century BC usually wore a simpler cuirass that appears to be made of linen or leather, with bronze equipment becoming increasingly rare, the prerogative of prominent heroes in mythological scenes. By the late 5th and early 4th century, paintings and reliefs show ordinary hoplites wearing no body armour at all. Sometimes they wear some thick cloth over their tunics, but often they lack even such basic protection. This development is confirmed by Classical literary sources suggesting that the average hoplite around 400 BC wore no armour except a simple helmet, and relied for his defence primarily on his large round shield. For example, the 11,000 hoplites of the mercenary army called the Ten Thousand do not appear to have had much more than 50 cuirasses between them.
Unfortunately, our literary sources do not actually discuss the topic, and so we are left guessing what purpose the lightening of the panoply served.
Anderson himself argued that hoplites felt a need to become more mobile, since they were increasingly fighting light-armed opponents, starting of course with the Persians during the Persian Wars (which happened right around the time the "lightening" becomes notable in the artistic record). This idea was no doubt prompted by numerous descriptions from Classical sources of hoplites trying and failing to catch skirmishing light-armed troops in pursuit. However, there are two problems with this theory. First, the Persians did not skirmish. Persian line infantry, even though its primary weapon was the bow, never ran from an advancing enemy; they stood and fought him with spears, swords and axes. This enemy should not have inspired a lightening of the panoply. Indeed, Herodotos states that the Greeks had the advantage in close combat with the Persians precisely because they were more heavily equipped; it seems absurd for them to conclude that the way forward would be to discard that advantage. Second, all the accounts in Thucydides and Xenophon of hoplites being exhausted and finally overcome by light troops show that no lightening of the panoply would ever have given hoplites a chance against missile troops. As long as they carried their big round shield, which defined them as hoplites, they would never be quick enough to overtake light troops in the chase (with the notable exception of young Spartans, who allegedly managed this a few times). If this was the reason for hoplites to shed their armour, it was both pointless and counter-productive, since the radical lightening of the panoply made hoplites more vulnerable to missiles.
We can also dismiss the theory that it was intended to allow the equipment of large numbers of warriors on the cheap, since hoplites were not normally equipped by the state. With some exceptions (mostly related to the equipment of mercenaries or to benefactions from wealthy citizens), the authorities did not concern themselves with the supply of arms and armour. Greek city-states had no military; citizens were called up for military service at need, and brought such weapons and equipment as they themselves could afford. It is not until Philip II's reform of the Macedonian army that we see the low cost of linen armour being regarded as an advantage in quickly raising a large body of heavy infantry - but this is notably because Philip was upgrading light troops, not downgrading hoplites (if I may be permitted the use of some computer game terms).
Both of these flawed arguments come together in the questionable story of the supposed reforms of Iphikrates. This Athenian general was thought to have re-equipped his hoplites as peltasts, a lighter and more flexible form of infantry. The process involved, among other things, replacing their bronze body armour with the linothorax. However, the story is found only in sources written in the Roman period, whose authors seem to have no longer understood exactly what either a hoplite or a peltast really was. The narrative is full of contradictions and confusions, and does not seem to be reflected in any part of Iphikrates' military career. It ignores the fact that both the peltast and the linothorax had existed for centuries before Iphikrates supposedly introduced them. It does not correspond in any way with the artistic trends observed by Anderson. In the debate about the lightening of the panoply, it is a useless distraction.
So, if not to create a lighter form of hoplite or to allow for cheap recruitment, what is the reason for the visible shift? For this we need to consider the major socio-economic change that occurred in Greece in the late 6th century BC. An explosion in land use and overall wealth suggests that more and more people were able to afford to arm themselves as hoplites; this is confirmed by the appearance of hoplite armies numbering in the thousands, first seen in literary sources around the same time. These large hoplite armies would have included many men who could afford only the most basic elements of hoplite equipment (the shield and spear) - creating a wholly new image of the hoplite from the older, heroic, conspicuously wealthy, aristocratic warrior. Artistic evidence for a "lightening" of the panoply may simply be the result of a trend to depict the average rather than the heroic hoplite. If this is right, the panoply was not actually getting lighter; artists were simply paying more attention to the large majority of men who were not decked out in full bronze.
Initially, the old leisure class seems to have tried to outshine the new hoplites by piling on ever more flashing armour, turning themselves into fully armoured "men of bronze". However, in the final decades of the 6th century BC, they found a new way to distinguish the rich from the merely comfortable: cavalry. Horses were prohibitively expensive to maintain, and no upstart farmer could ever hope to acquire one. This was a more effective way for the rich to stand out. They stopped upping the ante in terms of hoplite armour, and put themselves on horseback instead. Throughout the Classical period, horse ownership and cavalry service is the mark of the leisured gentleman, while hoplite service was regarded as the generic form of military participation.
An additional reason for the disappearance of heavy armour among hoplites may be the fact that, as hoplite formations became more solid and better organised, individual armour became less of a necessity. Again, this would fit well with the date when cuirasses start to disappear. In the Archaic period, battle was fluid and combat chaotic. Missiles flew everywhere and men fought ahead of their comrades to display their personal valour. In such conditions, heavy armour was essential for the survival of rich warriors who fancied themselves heroes. From the late 500s BC, however, there are mass hoplite armies in Greece; around the same time, cuirasses become less and less common in art. As the hoplite phalanx establishes itself as the backbone of Greek battle lines, and hoplites become defined as a type of warrior who fights together with others in a regular formation, shields become the essential feature of the hoplite's equipment, because the line of shields must remain unbroken. As long as it does, there is little need for a hoplite to cover himself head to toe in stifling bronze.
Unsurprisingly, partly because of the background of the cavalry and partly because of its focus on javelin combat, horsemen generally remained heavily armoured, and even increased their armour throughout the Classical period. By the 4th century BC, bronze cuirasses were a feature of the horsemen. Among the hoplites, only prominent officers and generals may have worn them, along with any old-fashioned types who felt the need. Panoplies remained an important form of religious dedication - the Athenians required each of the subject states of their Empire to send a panoply to Athens each year, to be dedicated at the Panathenaia festival - and of elite display, but they were no longer seen as a typical feature of hoplite equipment. In the 330s BC, when the Athenians introduce a system of mandatory military training for all citizens, which ends with the official gift of hoplite equipment to each cadet, that equipment consists of a spear and shield - nothing more.