When Alexander III of Macedon began his reign in 336 BC, he inherited the most formidable military force in the Western world. Honed by decades of reform and conquest under Alexander‘s father Philip II, the Macedonian army had already reshaped the Greek world at the point of the sarissa. But as Alexander set his sights on the Persian Empire and beyond, his forces would undergo a transformation as dramatic as the scope of his ambitions.
Infantry: The Evolution of the Phalanx
The core of the Macedonian infantry was the phalanx, a densely-packed formation of soldiers wielding long spears. According to the ancient historian Diodorus Siculus, Philip II had doubled the length of the Macedonian spear to around 18 feet, creating the two-handed pike known as the sarissa (Diodorus 16.3.2). This innovation gave the Macedonian phalanx an unparalleled reach and shocking power, as demonstrated at the Battle of Chaeronea in 338 BC.
Under Alexander, the phalanx remained the backbone of the army but saw its structure and composition change over time. The elite infantrymen formerly known as the "Foot Companions" (pezhetairoi) under Philip were renamed the "Shield-Bearers" (hypaspists) and organized into three chiliarchies of 1,000 men each (Arrian 1.14.2). The term pezhetairoi was now applied to the entire phalanx, which was divided into several taxeis of roughly 1,500 men each.
As Alexander‘s campaign wore on, however, the strictly Macedonian character of the phalanx began to change. By the time of the Indian campaign in 326 BC, Alexander‘s army had been fighting for almost a decade and suffered heavy casualties. Arrian reports that during the Battle of the Hydaspes against King Porus, the Macedonian losses were so severe that Alexander "sent back to Macedon those Macedonians who were too old for service or who seemed unlikely to remain fit for very long" (Arrian 5.19.4).
To replenish his depleted ranks, Alexander increasingly relied on soldiers recruited from his new Persian territories. According to Plutarch, in 324 BC at the great military review in Susa:
"The Macedonians were shocked to see Persians being introduced into units with Macedonians, but Alexander reminded them of the trust and partnership that now existed between the two peoples thanks to their shared struggles and his own personal example." (Plutarch, Life of Alexander 47.3)
This ethnic integration reached its height with the creation of the Epigoni ("Successors"), a corps of 30,000 Persian youths trained in Macedonian warfare. As historian Waldemar Heckel notes, "The Epigoni represented a bold attempt to broaden the ethnic base of the phalanx and bind Alexander‘s empire together through military service." However, many Macedonian veterans resented serving alongside Persians and revolted at Opis in 324 BC, forcing Alexander to ultimately discharge them (Arrian 7.8-11).
Cavalry: From Companions to Comrades
Just as vital to Alexander‘s success was his superlative cavalry. The elite Companion (Hetairoi) cavalry, composed of Macedonian nobility, was the hammer to the phalanx‘s anvil. Armed with lances and swords, these versatile horsemen could deliver shock charges, pursue broken enemies, and strike deep into hostile territory.
Initially organized into regional squadrons (ilai) of around 200 men each, the Companions underwent significant reforms under Alexander. After the execution of their commander Philotas in 330 BC, Alexander divided the Companions into two sections under his friends Hephaestion and Cleitus. He also appointed officers called lochagoi to lead smaller sub-units of 50-100 men, diluting the influence of the powerful ilarchs (Arrian 3.27.4).
Over time, Alexander steadily expanded and diversified the Companion cavalry. By the end of his reign, it had grown from 1,800 men to as many as 7,000, with new hipparchies recruited from across the empire (Arrian 7.6.3). Arrian states that at the Susa review, "the cavalry divisions were no longer exclusively Macedonian but contained horsemen from the subject nations" (Arrian 7.6.4). This included a 1,000-strong hipparchy of Central Asian nobles, possibly from Bactria and Sogdia.
Meanwhile, the Macedonian light cavalry known as prodromoi ("runners ahead") disappears from the historical record after 329 BC. It‘s unclear if they were disbanded or absorbed into the expanded Companion cavalry. In any case, Alexander increasingly relied on light horsemen recruited from his new territories, such as the Dahae horse archers from modern Turkmenistan (Arrian 3.28.10).
Tactics and Strategy: Adapting to New Challenges
Alexander‘s army didn‘t just change in structure and composition; it also evolved its tactics and strategy to overcome the challenges of conquering the largest empire the world had yet seen. Key to this was the army‘s ability to adapt to vastly different terrains, climates and enemies.
In the early battles against the Persians at the Granicus and Issus, Alexander employed the tactical system honed by Philip: a massive cavalry charge on the right flank combined with an oblique infantry advance. But at the decisive Battle of Gaugamela in 331 BC, Alexander faced a much larger Persian army on an open plain. As military historian J.F.C. Fuller explains:
"To win the battle Alexander had to throw his army obliquely across the front of the Persian host to take its left wing in flank. The maneuver was extremely hazardous, for a gap would open between Alexander‘s right and center. To cover this gap he posted the Agrianians and some cavalry behind his right, so that when the gap opened they could wheel into it." (Fuller, The Generalship of Alexander the Great, pp. 176-77)
The result was a stunning victory that left Darius III‘s army in ruins and opened the gates of Babylon and Persepolis. In later campaigns, Alexander adapted his tactics to the rugged terrain of Afghanistan and the Punjab. At the Battle of the Hydaspes in 326 BC, he made a daring river crossing to surprise the Indian ruler Porus, whose army included 200 war elephants. Alexander countered this menacing new adversary with the agility of his cavalry and light troops, baiting the elephants with javelins and arrows before letting them rampage through his open lines (Arrian 5.15-18).
Perhaps the greatest test of Alexander‘s army came during the grueling march through the Gedrosian Desert in 325 BC. According to Plutarch, the scorching heat and lack of water killed more Macedonians than all of Alexander‘s battles (Plutarch 66.1). Only the king‘s sheer willpower and improvisation, such as distilling seawater and marching at night, preserved a fraction of the army to reach Persia.
Legacy: A Military Revolution
After Alexander‘s untimely death in 323 BC, his generals divided his empire and continued to evolve the Macedonian military system. In the wars of the Diadochi ("successors"), the Macedonian phalanx and Companion cavalry remained the most potent weapons, but saw further specialization and variation.
For example, some of Alexander‘s veteran units received honorific titles and equipment, like the "Silver Shields" (argyraspides) who bore shields plated in silver. The Seleucid Empire, which emerged in Persia and Syria, fielded an elite cavalry unit called the "Royal Companions" (basilike ile), while also incorporating more heavily-armored horsemen known as cataphracts.
But perhaps Alexander‘s most enduring military legacy was the fusion of Macedonian and Persian traditions that he pioneered. As historian Albert Brian Bosworth observes:
"Alexander changed the face of the ancient world, and part of that change was to create a new type of army. He showed that a Graeco-Macedonian force could absorb substantial numbers of orientals and deploy them effectively on the battlefield. This was the bequest which Alexander left to his marshals and successors." (Bosworth, Alexander and the East, p. 258)
In many ways, Alexander‘s army was a reflection of his personal vision: a cosmopolitan force that united the best of East and West, Greek and Persian, Europe and Asia. It was an army that evolved relentlessly to surmount every obstacle in its path, much like the restless spirit of its commander. And though Alexander‘s empire would not long outlive him, the model army he forged would shape the art of war for centuries to come.
