regarding some earlier comments about the Plataia battle, the political/military events which took part before
Indeed the Macedonians were placed in the same flank with the medised Thessalians, Boiotians, Pierians and Perrabaians. However I dont recall reading anything about them taking part in the charge of the Theban cavalry (as mentioned somewhere) against the disorganised Megarians and Fliountians. Actually the Macedonian king had practically changed sides before the battle began hoping thant if the Greek alliance won they would help him get rid of the Persians back home. The Thebans had a really good cause to fight for in Plataia and that was the survival of their very own homeland which would lie under direct threat if the Athenian-Peloponnesian front would be victorious in Plataia. Thebai city themselves were spared from destruction after the Spartan intervention....that occured just for the simple reason that the Spartans were afraid that with Thebes destroyed, the Athenians would take advantage and become too strong too soon. So the Thebans got away only with the 2 leaders of the medised party of the city executed and with the Boiotian "Koinon" (league) dissolved. On the other hand judging from the attitude of the Macedonian king and general, the Macedonians would benefit from any outcome of the battle since they were cleaverly negotiating with both sides!
Had the Persians won the battle, the macs would upgrade their role in Paionia (who resisted the Persians), in Thrace and in Hellas. Had the Greeks fighting for the "greek cause" won, then the macs would get rid of the eastern occupation without any cost for them. That means they didnt have to give their best in the battle risking their lives, unlikely the Thebans who didn fought for their lives just outside their yards and were desperate to win. Moreover they were acting as some kind of intermediate so that the Boiotian cities (except Plataia and Thespiai which were Athenian and Spartan loyal allies respectable) were not harmed and also they were unsuccessfully trying to allure the Athenians in order to ally with Mardonios. That wasnt not achieved by military strenght of course but diplomatically.Alexander son of Amyntas was a political animal himself and a very ambitious and capable individual.
...although there are many info about the composition and the equipement of the Macedonian army of later times, few is known at the time of the "greco-persian" wars.
The Macedonian army should had fielded light infantry and skirmishers in Plataia as well. It seems to me that only the aristocracy was fighting on horseback, although there isnt any direct reference for this at the time. The cavalry was probable used to fight in coordination with the light infantry which backed them in many ways (somthing similar I guess with some fast moving non phalanx infantry units and "cavalry assistants" of Macedonian armies of later ages). We also know that the cavalry Macedon could field limited numbers since when Sitalkes invaded Macedonia with the encouragement of the Athenians, Thoukydides states that the Macedonians based their defence in their good quality cavalry or in withdrawing in the cities to be besieged by the enemy. Their cavalry was better than the Thracian one but much fewer in numbers. And maybe thats an indication that only the nobles could afford fighting mounted. Regarding their "nice breasts/cuirasses" I m not sure if Thouk had in mind any fancy looking cuirass. Actually the word he uses is "tethorakismenos" meaning "protected by cuirass". It doesnt provide any info about the material the cuirass was made or its style. Somebody could assume that since the cavalry constisted of wealthy macedonians, they could afford good quality cuirasses. However judging from all the neighbouring people, Thessalians, Thracians and Illyrians whose contemporary horsemen didnt seem to wear any heavy cuirasses made of metal (well with the exception of the king and generals themselves who usually imported such cuirasses from Hellas for themselves) I think that the Macedonian cavalry shouldnt be much different. Anyway that cavalry was superior to the Thracian one which was most likely composed by just mounted peltastai.
Those two Macedonian coins are aged around the lates 6th cent and the mids of 5th cent. The horsemen look much alike contemporary Thessalian horsemen depicted in pottery. They wear a kind of cloak and a petassoid helmet or hat. They bear two spears which are similar to the kamax spears suitable for fighting on horseback. Its not visible if they wear any cuirass at all. The horse itself doesnt seem to be protected by any "prometopis" or "episternidion"

The first important cavalry reforms we know came from the Thessalian Jason of Pherai (lates 5th cent) who used the "rhombus" formation and upgraded the cavalryneb defensive and offensive equipement. Until then the vast majority of the Thessalian and Boiotian cavalry wasnt heavily armoured. Jason lived later than Perdikkas but the Thessalian cavalry didnt consist only by the aristocracy. Even the "penestai" who were a similar caste with the helots of Laconia sometimes fought on horseback. We also know that Jason, whose state could field thousand of horsemen in the battlefield, had allied with the Macedonian and the Molossian king. Its uncertain however if the Thessalians copied the "heavy" cavalry from the Macedonians or vice versa.
The infantry could be armed with bows and javelins which the Macedonians used in their every day lives for hunting. Its uncertain if they were equiped with small "pelta" style shields since only the Thracians are recorded using such shields at the time...maybe they used hides and animal skins instead of decent shields
The light infantry seems to have been reformed in a kind of "hoplite copies" at the time of the reign of Archelaos. I guess they were not protected by heavy armour but resembled the Lacedaimonian outfit of the time. (simple "pilos" helmet, no armour, just the large hoplon shield which makes them hoplites)...well thats a speculation but there are not any real info available
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the requirement of timber for the naval program of Themistocles made him even more valuable to Athens as Macedonia was one of the few places which could provide it.
It was not only the Athenians who seek timber in Macedonia for their triremes but also Histiaios of Miletos and I think Polycrates of Samos earlier as well. The resources of Macedonia were invaluable, lets not forget the silver mines, the gold mines in Paggaion, the grain and the by land grain routes to Thrace ect