Greek Mercenaries in Alexander's army
It has been customary to simply equate mercenaries in Alexander's day with (traditional style) peltasts . However this then just begs the question of why are Alexander's mercenaries never recorded as fighting as skirmishers? Alexander used archers and (non-Greek) javelin men for this purpose, especially the Agrianians provided by his close friend King Langarus. In the open plains of Asia, javelin-armed skirmishers were ill suited to countering the Persian's most reliable military asset: their cavalry. In contrast, an Iphicrates-style 'peltast' with a long spear would be as effective as any other hoplite in warding off cavalry: the long experience of constant Greek and Persian confrontation had demonstrated the steadfastness of Greek hoplites in the face of mounted troops. As Alexander had ready access to non-Greek skirmishers, he had no need of traditional Greek peltasts. If the term mercenary had by Alexander's day become synonymous with peltast as understood by Xenophon and Thucydides, we would expect to hear of Alexander using his mercenaries in a skirmishing role, even if only on a single occasion. But we don't. The conclusion is to me inescapable: Alexander's Greek mercenaries were not such traditional peltasts. Instead he employed, in great numbers, the equally cheap Style-style hoplite which meant he had all the more men capable of standing up to his mounted Persian opponents.
One reason to discount large numbers of Alexander's Greek mercenaries being hoplites is Alexander's reluctance to use them in front line roles in battle. It is true that Alexander's historians focus on his activities to the exclusion of generals like Parmanio who often undertook side campaigns on their own, and that mercenaries often constituted a good proportion of the troops involved in such side expeditions. However, it is obvious from looking at the nature of Alexander's own such side expeditions that the nature of the fighting involved was often markedly different from that expected in a pitched battle; and even then there was usually a nucleus of Macedonian troops around which such expeditions under Parmenio, Krateros, etc. were formed, who were no doubt expected to bear the brunt of the fighting.
Hoplites, when well-trained and experienced, were formidable soldiers, and proved many times to be the match for even Macedonian heavy infantry. Two examples will suffice, others can be found: The Thebans in 335 BC, despite being considerably outnumbered, had the better of the Macedonians until Alexander committed his fresh reserve of foot guards, while part of Darius' Greek hoplite phalanx at Issos managed kill no less than 120 Macedonian officers before marching off the battlefield in good order as the rest of the Persian army was routed.
I have no doubt that if Alexander had sufficient numbers of such troops in his army he would have used them in an aggressive manner. Instead he used his mercenaries in the rear line.21 As Iphikratean hoplites, their long spears meant they could easily hold off Persian cavalry; had they been in the front line they would be very vulnerable to the other troop type the Persians disposed large numbers of: archers and other missile-men. Traditional hoplites had large shields covering their bodies, and greaves to protect their legs. Alexander's phalangites had smaller shields, but unlike Philip's original phalangites had acquired body armour, the thorax, in compensation, and additionally derived some protection from the forest of pikes sloping over their heads. Iphikratean hoplites would have none of these defensive benefits, and so could be expected to take heavy casualties from archery if they were to be positioned in the front line; being in the second line, their most likely opponents would be cavalry outflanking the main formation, as happened at Gaugamela. Alexander's tactical dispositions are entirely sensible if these considerations are taken into account; it is hard to reconcile them if most of his Greek mercenaries were either traditional hoplites or traditional peltasts.
Iphicrates and the Macedonian Phalanx
It is often remarked how similar the equipment of an Iphikratean hoplite is to that of a Macedonian phalangite. Philip is credited with inventing the equipment as well as the order of the Macedonian phalanx in or soon after 359 BC, which should probably be taken as a reference to the invention of sarissa (pike), since the other items of equipment said to be carried by Phillips' men by Polyainos, helmets, greaves and shield, were clearly already in existence, even in Macedonia. The spear of a hoplite is usually reckoned as being somewhat under 6 cubits long, about 8 feet. By Diodoros' account, the Iphikratean spear would thus be 8 cubits long, or 12 feet, and hence still shorter than a Macedonian sarissa which had a minimum length of 10 cubits, 15 feet, and was normally 12 cubits or 18 feet long, although later Hellenistic phalanxes apparently used pikes 14 cubits long. By Nepos' somewhat less reliable account, the spear would be about 16 feet long which would put it in the sarissa range, although it is hard to see how such a weapon would be wielded one handed, and if it was wielded two handed, Iphicrates would be the inventor of the phalangite, not Philip. It is more likely Nepos' source considered that a normal hoplite's spear was less than 8' long and indeed many artistic depictions show hoplite spears 7 foot long or even slightly less, which would put the Iphicrates spear at under 14 feet long.
It is widely appreciated that Philip is likely to have picked up many of his tactical ideas from living for a time in the house of Pammanes, close friend of that great tactical innovator, the Theban general Epaminondas. It is not generally appreciated however that Philip was even more closely related to Iphicrates, who was Philip's own brother by the adoption carried out by Philip's father Amyntas. Philip could not but help but have been aware of Iphicrates' reforms.
The Macedonian army before Philip's time relied on its aristocratic cavalry. It included a few hoplites, but the majority of the men were essentially an ill-armed and untrained rabble. Thucydides implies that the infantry situation improved somewhat just before the start of the 4th century, but it is evident from the number of times the Thracians and especially the Illyrians overran the country over the next 50 years that they were still not up to the task of defending the borders, let alone catapulting Macedonia onto the world stage.
Philip took Iphicrates' reforms as his model and adapted them to his own needs. He needed to equip himself with an infantry force that could fight competently in hand-to-hand, in a phalanx, and to do so as cheaply as possible since he would have to pay for it personally, rather than his infantrymen, who being essentially peasants, not middle-class city dwellers, could not possibly afford to do so themselves. Iphicrates had pointed the way. The Macedonians were already using a bronze shield before his time, but it was not the aspis of the Greek hoplite, as it was smaller, between 60 and 75 cm in diameter, and lacked the characteristic rim of the Greek aspis - in other words, something of a hybrid between the traditional pelta and the Argive aspis. It was probably introduced by Archelaos, who sometime between 413/2 BC and 400/399 BC according to Thucydides (2.100) "reorganized the cavalry, the arming of the infantry, and equipment in general", and the first depictions of it indeed come from circa 400 BC. It may have been adapted from the neighboring Illyrians. The southern Illyrians bordering Macedonia used round shields that are extremely similar to those used by the Macedonians.
While such shields might have been equipped with an Argive-style shield grip, their smaller size meant that the position of the forearm brace would have been different from that in a Greek aspis. Equipping such a shield with a Greek-style grip in the same relative positions as a Greek aspis rather than a pelta, so that the forearm brace retained its position near the center of the shield, would mean that the hand would be positioned much closer to the edge of the shield. This would normally be a disadvantage in terms of balancing the shield and protecting the hand, but it would allow the hand to grip a spear despite the curvature of the shield. This I believe was Philip's first military innovation: providing the Balkan bronze pelta with a shield grip in the Greek manner positioned so that the hand was right at the rim of the shield. The length of the Greek spear, as carried by Iphicrates' men, had been limited by the requirement for it to be wielded in one hand, but with a two handed grip it could now be lengthened even further and become a true pike. Providing his army with even these arms would have overtaxed the finances of the country at the time, and no doubt only officers got the full kit of greaves, pike, bronze shield and helmet. Even they would not have had armour for the torso, and rear rankers probably had to be content with bare legs or boots (krepides), a cheap mass-produced wicker pelta and a helmet fashioned from leather.
Unlike a spear, which retains some utility in single combat, a pike is essentially useless outside a compact phalanx. The formation, in both senses of the word, of the Macedonian phalanx, gave Philip an infantry force that was capable of standing up to Greek hoplites in open battle. If it was to retain any strategic utility however, its men needed to be able to fight outside the confines of the phalanx. As with most peoples living in an area surrounded by hills, the traditional Macedonian weapon was the javelin. Philip ensured that his men were trained in the use of both weapons, and carried whichever was the most appropriate for the occasion, so that his infantry could fulfill the role of both hoplite and peltast as need be. When marching through broken country, javelins were carried: Polyainos relates how when Onomarchos' Phocians ambushed Philip's men, they were able to fight back at a distance. Similarly, a pike was of little use when assaulting a city, when troops had to climb ladders up walls and inside siege towers, so the javelin was carried in this situation as well.
The role of the Hypaspists (shieldbearer) was to maintain the linear connection between Alexanders advancing right wing and the right flank of the Phalanx. Since this force was designed to spread out, it was organized into smaller tactical units than a Phalanx. The exact armaments of a Hypaspist are unknown but they appear to be Hoplite armed heavy infantry with lighter body armor for increased mobility.
Philip's brutally efficient training programme, backed by his autocratic royal power, ensured his men lived up to his expectations. Training men to use two sorts of weapons with equal facility is no easy task, and very few other classes of warriors over the millenia have ever attained such dexterity; the few that readily spring to mind are mostly aristocratic steppe horsemen accustomed to both lance and bow. Training his men to use two weapons that required a completely different formation to fight with, a rigid pike phalanx against the loose order required to hurl javelins, made the achievement all the more outstanding, especially given the inclusive nature of his reforms - it was the entire national levy that was so trained, and not just a picked elite. The result was that not only could Philip eventually come to count on troops as good as any opposition could field, but he would have numbers of his side as well.
This then was the force that Alexander inherited from his father for the conquest of Persia. The evolution of the Macedonian infantry proceeded under Alexander. As already noted, the phalanx had acquired body armour by the siege of Tyre at the very latest, possibly non-metallic, as it was burnt when it was replaced with elaborately decorated cuirasses while in India. Expansion of the army entailed reorganization changes, and at his death, Alexander was experimenting with a radically new type of phalanx, incorporating javelin men and archers in its rear ranks, but it was never used in action. Upon his death, his generals carved his empire up amongst themselves. Quality troops were at a premium, and no-one could afford to dilute the effectiveness of their most valuable units with such Persian missile troops.