What made Alexander the Great’s army undefeatable?

Alexander the Great inherited a Macedonian army that already possessed a formidable structure thanks to the recent military reforms of his father, Philip II.

When Philip had come to the throne in 359 BC, Macedon had been weakened by war and internal strife.

Recognizing the need for a strong military to secure his throne, he embarked on an ambitious program of military innovation.

This was inspired by his experiences as a hostage in Thebes years before, where he observed the effectiveness of the famous Theban phalanx. 

Philip's reforms began with the reorganization of the Macedonian army into a more structured and disciplined force.

The new Macedonian army would become the first professional army in world history.

This meant that soldiers were trained year-round: a departure from the traditional Greek practice of citizen soldiers who farmed during peacetime.  

Next, Philip focused on developing specific troop types that were expected to fulfil dedicated tasks in battle.

The most famous of these were the pezhetairoi.

One of Philip's most significant changes to the old army was the development of the new form of Macedonian phalanx.

Philip called the soldiers in the pezhetairoi, or ‘foot companions’. They stood in ranks typically eight men deep, with the first five rows holding their spears forward to reach the enemy simultaneously.  

This was a tightly organized infantry formation of spearmen that utilized a revolutionary new spear, called the sarissa.

They were about 18 to 22 feet (6 meters) in length, which was much longer than the traditional hoplite spears used by other Greek city-states.

This allowed the Macedonian soldiers to engage the enemy from a further distance and giving them a significant advantage.

As a result, the sarissa created a formidable wall of pikes that was nearly impenetrable when maintained.

This formation was particularly effective in open battlefields where it could be fully deployed without geographical constraints. 

However, the phalanx required a high level of coordination and cohesion among its members to be most effective.

As such, any combat maneuvers were executed as a unit, in order to maintain the alignment and spacing critical to the defense offered by the overlapping sarissas.

In battles like Chaeronea in 338 BC, the phalanx proved its superiority by breaking through the enemy lines with its sheer force and the disciplined precision of its advance.  

Alongside the updated phalanx, Philip also expanded and modernized the Companion Cavalry.

Known as the Hetairoi in Greek, this was the elite cavalry unit of the Macedonian army, which consisted mainly of Macedonian nobility.

They were already well-armored and rode on specially bred horses. Usually, they were armed with a long lance called the xyston. 

The Companion cavalry was often used in flanking maneuvers that were designed to break enemy lines and cause chaos within opposing forces.

Under Philip’s reforms, the Companions grew from 600 men to 3000. They quickly became the lynchpin of Alexander’s aggressive battle tactics.

The Macedonian king would personally lead them in do-or-die charges to seize decisive victories. 

The Companion’s mobility allowed them to quickly exploit the openings they created, turning the tide in favor of Macedonian forces.

For example, during the Battle of Issus in 333 BC, Alexander was at the head of the Hetairoi in a bold charge against the Persian king, Darius III.

Their attack disrupted the Persian command structure and caused widespread panic; Darius barely escaped with his life.  

Philip also developed an elite infantry unit known as the Hypaspists. The meaning of this term is somewhat unknown but is often translated as "shield-bearers".

In reality, they were similar to the traditional phalanxes used by other Greek armies.

They were positioned to fight alongside the more heavily armored phalanx and the agile Companion cavalry. 

Serving as a bridge between the heavy infantry and cavalry, the Hypaspists were essential in maintaining the cohesion of Alexander's forces during complex maneuvers.

They were often deployed to exploit breaches in enemy lines created by the phalanx or to cover the flanks of the cavalry during charges. 

They often served as his personal guard to Philiip and Alexander in battle. The unit was typically composed of approximately 3,000 men, handpicked for their skills and loyalty.

Their training emphasized speed, endurance, and combat versatility, which enabled them to execute a range of tactical roles on the battlefield, from skirmishing to forming defensive squares against cavalry attacks. 

Through these wide-sweeping reforms, Philip had successfully transformed the Macedonian army into a force that was capable of conquering and holding vast territories.

His greatest legacy when he died in 336 BC, was that his son, Alexander the Great, had instant access to a well-oiled military machine that was ready to build an empire stretching across three continents. 

The Battle of Gaugamela in 331 BC is often hailed as the prime example of how effective all three key military units - the pezhetairoi, the Hetairoi, and the Hypaspists -  were on the day of battle.

This was the final confrontation between Darius III of Persia and Alexander, which would decide the fate of the entire Persian Empire.

However, Darius appeared to be in a much stronger starting position. He outnumbered Alexander’s army almost four-to-one and had deployed both war elephants and scythed chariots. 

To achieve victory, Alexander ordered the pezhetairoi to hold the center with their dense and disciplined phalanxes.

Then the Hypaspists were given the critical role of protecting the vulnerable flanks of the phalanxes to maximize the time they could hold out against the Persian infantry.

When Darius sent his chariots forward, the Hypaspists adapted quickly to neutralize them. 

While this was happened, Alexander led the Hetairoi to the far right flank, drawing the dangerous Persian cavalry after him.

Not only would this draw them away from the phalanxes, but it created a gap in the Persian lines.

Then, at a pivotal moment, the Hetairoi were sent on a fierce charge into this gap.

With Alexander at the front, the Companion Cavalry easily broke through the line, and headed directly for Darius III.

The Persian king panicked, fled the field, and the entire Persian army collapsed. 

Each element of the Macedonian force had played their part perfectly: the pezhetairoi held the line steadfastly, the Hypaspists secured the flanks and addressed any threats of encirclement, and the Hetairoi executed a decisive charge that led to the rout of the Persian army.

The seamless integration of these units under Alexander's command achieved an almost impossible victory. 

As Alexander the Great's conquests spread from Greece to the edges of India, the integration of foreign troops became a park of his overall military strategy.

Alexander often enlisted soldiers from the defeated regions, notably Persians, Bactrians, and even Indians, who brought with them a range of martial traditions.

These new recruits became auxiliary forces, fighting alongside Macedonian and Greek soldiers. 

By the time of his campaign in the Punjab region around 327 BC, Alexander had formed a new unit, the "Epigoni," composed primarily of these foreign warriors.

This unit was instrumental in maintaining order in newly conquered territories and in reinforcing Alexander’s ranks during his return to Babylon.

This concept, known as the "heterogeneous army," allowed for a combination of tactics and weaponry that could be adapted to diverse combat situations. 

The proactive integration of different cultural groups was aimed at helping quell resistance and promoting loyalty among his forces. 

Alexander wanted to lay the groundwork for a new era where the blending of different cultural practices became a source of strength rather than division. 

Alexander the Great's army, which could number up to 40,000 soldiers and 6,000 cavalry at its peak, required an incredibly efficient system to ensure a steady supply of food, weapons, and other necessities.

To manage this, Alexander established mobile supply depots and relied on a network of local resources.

These were then systematically requisitioned and distributed as his army advanced. 

In addition to these provisions, Alexander employed a dedicated corps of engineers, blacksmiths, and veterinarians who traveled with the troops.

They ensured that the army's logistical needs were met promptly, regardless of their location across the growing Macedonian empire.

When this system worked well, Alexander could execute prolonged campaigns in distant lands without the risk of supply shortages.