All Hellenistic armies maintained the basic order bequeathed to them by Alexander. Their core was formed by the ordinary troops of the infantry phalanx linked to other formations. On the right they were linked to a unit of elite phalanx infantry equivalent to guards units called by various titles connected with the characteristics of their shields. At various times they are known as either the hypaspists, silver shields or bronze shields. One subunit consisted of the royal bodyguard, the Agema, a title also used in Alexander’s army late in his reign. That elite formation linked the infantry to the elite Companion cavalry units, including the royal horse guard, on the right where the king was often stationed. The other wing was also formed of cavalry, with light-armed infantry and horse also posted on the flanks or to the front as flank guards and to provide for an elastic offensive and defense. The close imitation of Alexander’s model is clear. The increasing use of foreign mercenaries must have made the appearance of these armies far more diverse than earlier formations. One new development was the use of true reserve troops behind the lines.

There was little change in tactics in the three centuries after Alexander. For about a century the basic goal of the commander was to create a gap in the enemy army, as at Paraetacene, or to strip the infantry of its cavalry support by driving it off. The cost of training and maintaining cavalry gradually led to the growing importance of the infantry phalanx. Later battles were often slogging matches between phalanxes, as at Sellasia in 222 or Raphia in 217. A feature retained from the Classical period and transmitted through Alexander was the notion of an offensive and defensive wing. The destruction of Demetrius’ offensive cavalry wing at the battle of Gaza in 312, despite a shield of light-armed troops, ended his chances of success.

The general characteristics of large set-piece battles in the Hellenistic period are remarkably stable. The armies were homogeneous in equipment and training if not in ethnic composition. As in siege warfare, there was some technological experiment, especially with the use of elephants, but the older arms remain decisive.

CAVALRY

Given the expense and difficulty of maintaining a cavalry force, the ratio between cavalry and infantry constantly decreased in the course of the period in the major Hellenistic armies. It fell from about one to five after Alexander to about one to eight by the end of the third century. The highest figure we possess is a combined force of about 21,000 cavalry out of a total of all armed forces of 155,000 at Ipsus in 301. Heavy cavalry gradually changed its role from that of deciding the battle to one in which its main task by the later Hellenistic period, as at Raphia, was to defeat the opposing cavalry and ensure that the infantry could advance unmolested. Cavalry still had the same uses as it had in the Classical period. It still was unable to face unbroken heavy infantry and would only be able to do so after the technological advances that resulted in the stirrup and an appropriate saddle as well as larger horses. Light cavalry was also used in this period as a flank guard or screen and to provide an elastic defense, an advance owed to Alexander and visible already at Gaugamela.

281. Polybius, Histories 12.18.2–3=Callisthenes FGrH 124 F35
We know very little about Hellenistic cavalry formations. The largest unit seems typically to have been the hipparchy, as it was in the later army of Alexander, and below that came the ila. The smallest unit was the oulamos. If we accept Polybius’ figures it may be that an ila contained 128 men. These calculations are part of Polybius’ criticism of an earlier historian’s account of the battle of the Issus.

There were thirty thousand cavalry, as Callisthenes himself says, and thirty thousand mercenaries. It is easy to figure out how much space is required to contain them; for the majority of cavalry are arrayed eight deep for real effectiveness and between each ila there needs to be a space equal to the frontage of the troop to allow them to wheel and to easily face about. 

282. Polybius Histories 3.117.4–5
Polybius is referring to the cavalry flank and rear attacks on the Roman force that completed the encirclement of that force and assured its defeat. It is the vulnerability of isolated heavy infantry that is uppermost in the historians mind.

All the rest [of the Romans], about seventy thousand, died bravely. At this battle [Cannae] and in earlier actions their cavalry was crucial to the Carthaginians. This made it clear to posterity that it is better to give battle with half as many infantry as the enemy but to have an overwhelming superiority in cavalry, than to go into battle evenly matched in all respects.

MACEDONIA

With the establishment of the Antigonid house in Macedonia, quiet finally settled over a state that had been under almost constant military pressure since Alexander. Perhaps half of those eligible for military service had been swept away in the fifty years since Alexander’s death in war or through emigration.

The Macedonian kings were conscious of the need to husband their manpower and hired mercenaries not only for long-term garrison duties but also as expendable substitutes for their own forces. A number of treaties are extant between Cretan cities and the Macedonian kings arranging for mercenary service. The use of mercenaries was limited by the slender resources of the Antigonids in comparison to their rivals. The national army had a fairly constant number of effectives of around twenty-five thousand. The officer core was still composed of Macedonian nobles. Like the Aetolian and Achaean Leagues, its major rivals, it possessed a citizen army as its core. The destruction of the Macedonian monarchy by the Romans in 167 was more the result of superior Roman manpower and than of any decrease in the effectiveness of Macedonia’s army.

301. Diodorus Siculus, 18.12.2
The movement referred to in this passage is a rising of many of the Greek states under Athenian leadership in the wake of Alexander’s death, the Lamian War. It is crucial to note that the League’s early successes appear to have been due to the lack of manpower experienced by the Macedonian regent Antipater. It was the return of time-expired Macedonian veterans that made the eventual Macedonian victory in the next year possible.

When [Antipater] learned of the concerted movement of the Greeks that had taken place against him, he left Sippias as general of Macedonia, gave him enough troops and ordered him to conduct a levy of as many as possible. He assembled his army of thirteen thousand Macedonians and six thousand cavalry. (At this time Macedonia lacked citizen-soldiers because of the great number that had been sent off to Asia as replacements for the army.)

302. Diodorus Siculus, 20.110.3–4
The reference is to Demetrius’ Macedonian campaign against his rival Cassander in 302. It ended in a compromise because of his need to come to his father’s aid in Asia. The passage illustrates what appears to have been a fairly typical Macedonian force in the years after Alexander death.

As Cassander saw that Demetrius’ affairs were going forward according to the latter’s plans, he more strongly garrisoned in advance Thebes and Pherae and gathered all of his forces and encamped opposite Demetrius’ army. Cassander had a total of 29,000 foot and 2000 horsemen. Demetrius had 1500 cavalry and not less than 8000 Macedonian infantry; his mercenary forces was 10,500, there were also light-armed formations and all types of adventurers who had assembled for war and pillage and who numbered more than 18,000; the resulting grand total was about 56,000 foot.