Why did Alexander the Great invade Persia?

Alexander's invasion of Persia

Upon arriving on the shores of Persian held territory, Alexander the Great hurled his spear in Homeric fashion to show ‘that he received Asia from the gods as a spear-won prize’.

This prize had been made accessible by the inheritance of his father, Philip II of Macedon, whom left behind a professional standing army, an ideological pretext and an expeditionary force already in the field.

As Captain-General of the league of Corinth, imbued with the romantic conceptions of a second Achilles, Alexander would lead the Greeks against the barbarians in a war of vengeance.

However, the reality is far less glamorous, Alexander and his companions set out to invade Persian territory for territorial expansion and profit.

For Alexander himself, the invasion had held further promise, a chance to outshine the achievements of his mythical ancestors and his greatest competitor, his father.

This article will explore these propositions in an attempt to better understand the motivations that brought Alexander and his army into Persian held territory.

The Military Inheritance from Philip II (means)

The inheritance granted to Alexander had provided the means for an invasion of Asia Minor.

Alexander inherited a professional standing army with twenty years of cumulative combat experience and training.

Philip was training the Macedonians before the dangers. He made them take up their arms and march often 300  stades carrying helmet, shield, greaves, sarissa, and in addition to their arms rations and all gear for day-to-day existence. (Polyaenus, Stratagems 4.2.10)

Philip was not panic-stricken by the magnitude of the expected perils, but, bringing together the Macedonians in a series of assemblies and exhorting them with eloquent speeches to be men, he built up their morale, and, having improved the organisation of his forces and having equipped the men suitably with weapons of war, he held constant manouvres of the men under arms and competitive drills. He devised the compact order and the equipment of the phalanx, imitating the close-order fighting with overlapping shields of the warriors at Troy, and was the first to organise the Macedonian phalanx. (Diodorus Siculus 16.3.2)

Philip also provided new military tactics involving the coordination of combined cavalry and phalanx warfare.

At the battle of Bardylis in 359 B.C., an evenly matched set-piece battle against the Illyrians featured a concentrated phalanx assault on the right that exposed a hole for the cavalry to charge through.

Bardylis, relying upon his previous victories and the gallant conduct of the Illyrians, came out to meet the enemy with his army; and he had ten thousand picked infantry soldiers and about five hundred cavalry. When the armies approached each other and with a great outcry clashed in the battle, Philip, commanding the right wing, which consisted of the flower of the Macedonians serving under him, ordered his cavalry to ride past the ranks of the barbarians and attack them on the flank, while he himself falling on the enemy in a frontal assault began a bitter combat. But the Illyrians, forming themselves into a square, courageously entered the fray. And at first for a long while the battle was evenly poised because of the exceeding gallantry displayed on both sides, and as many were slain and still more wounded, the fortune of battle vacillated first one way and then another, being constantly swayed by the valorous deeds of the combatants; but later as the horsemen pressed on from the flank and rear, and Philip with the flower of his troops fought with true heroism, the mass of the Illyrians were compelled to take hastily to flight. (Diodorus Siculus 16.4.4-5)

Lastly, the development of the sarissa-phalanx (or Macedonian phalanx) that involved a Hoplite formation using the 15-18 foot Sarissa. The reach of the sarissa provided devastating results for frontal assaults on favourable ground. For a more detailed explanation of the sarissa, see our article on Macedonian Sarissa, Form, Function & Origin.

The Macedonian Phalanx using the Sarissa

Financial Necessity (motive)

The endowment of the substantial military resources of Macedonia meant the inheritance of the financial constraints that are imposed by those same resources.

The war economy of Philip had left Alexander with a depleted treasury, a point highlighted at the mutiny of Opis.

I inherited from my father a few gold and silver cups, and less than 60 talents in the treasury; Philip had debts amounting to 500 talents, and I raised a loan of a further 800.

Fortunately for Alexander, the revenues of Macedonia were substantial,  but the necessity for Alexander to borrow funds highlights an economic problem, the revenues of Macedonia were not covering the cost of the army.

It is likely that Philip had sought the invasion of Persia to complete a mastery of the Aegean, tribute and plunder from the conquest used to compensate the army.

The untimely death of Philip had delayed the invasion and uprisings among the Greek city-states had delayed it further.

These events left Alexander with a decision, either to reduce the financial obligations through the retrenchment of soldiers or increase revenue through a profitable war of aggression.

For Alexander, the choice was a timely invasion of Persian territory for profit.

The Warrior Ethos (motive)

It is with greater delicacy that we should treat the personal motives of Alexander, the man that Arrian had surmised as in ‘competition with himself if there was no one else’.

A product of his environment in Macedonia, Alexander brought the nephew of Aristotle along to immortalise his exploits, highlighting the emphasis he placed upon his own achievements.

Tutored by Aristotle, Alexander claimed to be a descendant of the mythical figures of Heracles and Achilles.

For Alexander, his father had been his greatest competitor; the man whom he criticised would ‘leave no great or brilliant achievements to be displayed to the world’.

The successful invasion of Persia would dwarf even the substantial achievements of his father.

The (un)Timely Death of his Father (Opportunity)

The succession of Alexander to King of Macedonia was not a foregone conclusion and shackled Alexander with a political debt.

Only through a virtue of proximity to the court and the support of Antipater allowed Alexander to eliminate or pacify rival factions.

A key figure for pacification was the elder statesmen Parmenion, whom extracted a heavy price from the would-be king.  The price included the appointment of many of his friends to positions of influence, so much that Justin quipped that the staff headquarters for the invasion looked ‘more like the senate of some old-time republic’.

It is from this weak point of succession that we must consider that Alexander wasn’t the only influencer on the decision to invade. Many of the senior staff had served under Philip, were involved in the expeditionary force and readily lent money to Alexander for the invasion. These statesmen would profit greatly from an invasion of Persian territory and Alexander owed them both a political and financial debt.

An Ideological Pretext (Opportunity)

It is at this point we shoulder consider the ideological motives espoused by Philip and Alexander as the pretext for the invasion.

The notion of the invasion of Persia hadn’t been a new concept, ever since Aristophanes’ Lysistrata, a dream existed of uniting warring Greek states against the barbarians of the east.

Isocrates had stressed the inherit weakness and abundant wealth of Persia as an inducement to invasion, points highlighted when he wrote to Philip.  Philip took the advice literally and used the war of vengeance as an instrument of policy for a Macedonian led invasion.

Alexander continued in these footsteps, and literally the footsteps of Xerxes’ as he retracted the steps of the Persian invasion in reverse direction.

The problem was the actions didn’t match the rhetoric, Greek troops made up only a tiny fragment of what was available and they lacked the enthusiasm to support a figure that had brutalised them.  Once Alexander had secured Asia Minor, the propaganda largely disappears from the record and upon the news of Alexander’s deaths, the Greeks promptly broke out in rebellion.

A Panhellenic crusade, it was not. Wonderful propaganda? I think so.


In the spring of 334 BC, Alexander crossed the Hellespont into Persian held territory.

At the head of a superficial Panhellenic invasion force, the King of Macedonia marched under the pretext of a crusade of vengeance.

However, the reality is far less romantic, Alexander and his companions set about the invasion in the pursuit of less ideological motives, that is, territorial expansion and profit.

For Alexander himself, the invasion provided an opportunity to outshine his father’s achievements, to emulate the heroic past of Homer and cast his own indelible shadow over future successors.

It is this motive that would drive Alexander through Persian territory  and the rest is history.

Scott McCulloch
Scott is a web programmer, author and ancient history student. He is the founder of Ventrian - a provider of DotNetNuke Modules and Ancient Life - an Ancient Greek & Roman history blog

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7 Comments on "Why did Alexander the Great invade Persia?"

  1. William Bor says:

    I agree with the economic motive ,how else could or would his army march from macedon to india.

    I am exploring the relationship between the idea of philip’s group , the one alexander inherited and the economic motive

    interested ?

    • I agree – it was a war economy.

      But let us not also forget the role the historic writers played in this scenario. It was in there own narrative best interests to paint the picture of a rags to riches story.

      Consider the Persian envoy who is supposed to have visited Alexander as a boy, ‘This boy is the great king, ours is only rich’. We must treat some of the financial claims as sceptical, things not have been as dire as they seemed.

  2. Nick Harrison says:

    Well written and executed. Sums up both the rhetoric the invasion was based on and the reality of the situation, which was a combination of hubris and a means to compensate the war machine that Alexander inherited from his father.
    What fascinates me most about Alexander is that he never wanted to stop. I don’t get it. My understanding – and correct me if I’m wrong – is that the invasion of Persia ended up being highly profitable. After taking Babylon Alexander could have come home and enjoyed the thanks of a grateful nation. The Persian invader had been soundly beaten on their home turf. But, for whatever reason, Alexander continued to capture and in some cases – Tyre – destroy cities.
    Did Alexander desire world domination?

  3. Nimbot says:

    Just a question, what would some of his political reasons be? (aside from economic reasons)

  4. @Nick – In the months before Alexander’s death, he was making plans to continue his campaign. He might even have ended up in Italy which would have been interesting :)

    @Nimbot – Politics is quite often tied up with economics. Justin quotes that the invasion force for Persia looked ‘more like the senate of some old-time republic’. It would seem that the seasoned officers of the court had quite an interest in the campaign. Speculating, but maybe they thought that Alexander could be used as a pawn (how wrong they were!).

  5. Hannah says:

    Why did Alexander the Great start a war with the Persians.

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