Why Alexander the Great Had To Conquer the Persian Empire – 3 Reasons

Alexander the Great conquered the Persian Empire, that is pretty much common knowledge. But the 3 reasons why Alexander the Great had no other option but to conquer the Persian Empire are much less known. So let`s find out why Alexander the Great conquered the Persian Empire.

Alexander’s father Philip II had started the war with the Persian Empire when he invaded Asia Minor in the Spring of 336 BC. So Alexander the Great inherited the war in the fall of 336 BC and had no other choice than to continue it. Philip II had started the war because the Macedonian treasury was empty and conquering the wealthy (Persian) Asia Minor promised a rich war bounty. A victory over Persia also promised to legitimate Macedonian supremacy over Greece.

Let`s take a closer look!

Alexander the Great inherited the war against Persia from his father

The biggest reason why Alexander the Great attacked and eventually conquered the Persian Empire was that he had no other choice. Now that sounds kind of odd, one would think that invading a neighboring empire is somewhat of a conscious choice. And yes, the decision to attack the Persian Empire was a conscious choice.

It just wasn`t Alexander`s choice.

Alexander the Great only inherited the already ongoing war against the Persian Empire when his father, Philip II, was assassinated at the wedding of his daughter Cleopatra by his bodyguard Pausanias in October of 336 BC.  By the way, aside from the war with Persia Alexander the Great also inherited the kingship of Macedon, the position of Hegemon of the League of Corinth (a military alliance of almost all mainland and island greek city-states except for Sparta), as well as the army that Philip II had reformed and that Alexander would use with great success.

Philip II of Macedon, the father of Alexander the Great, had already started the war with the Persian Empire in early 336 BC when he sent a vanguard of 10,000 Macedonian soldiers and mercenaries under the command of the generals Parmenion and Attalos over the Hellespont into the Persian Asia Minor. He planned that he himself as well as the main force of the army would follow in the fall of 336 BC when the vanguard had already won over allies and established bridgeheads.

But that would not happen. First Philip II was held up by inner-political problems and then he was assassinated at his daughter’s (Cleopatra) wedding on 21 October 336 BC by his bodyguard Pausanias (who in return was killed by friends of Alexander the Great.

So when Alexander the Great claimed his inheritance and became king of Macedon and hegemon of the League of Corinth he had a big problem!

The Macedonian vanguard that had invaded Persian Asia Minor in early 336 BC was still there. And not only that, the generals Parmenion and Attalos had also been quite successful. And while they retreated to the Hellespont and limited themselves to only holding a couple of bridgeheads across the Hellespont when they heard of Philips’s death, they had still invaded Persian grounds.

Even if Alexander the Great would have immediately pulled all Macedonian troops out of the Persian lands of Asia Minor when he became king, the Persian king would have still not forgotten the fact that the Macedonians had invaded his Empire. He would have retaliated. So Alexander the Great had no other choice than to continue (and ideally win) the war with the Persian Empire.

Speaking of the Macedonian bridgeheads over the Hellespont. The Macedonian vanguard actually managed to hold these bridgeheads for almost two years until 334 BC when Alexander the Great had solidified his rule and could now turn towards the Persian Empire.

However, Alexander the Great was probably not too sad about having to continue the war with the Persian Empire since the two reasons why his father had started the war in early 336 BC were still there.

Persian Asia Minor was rich & the ideal target for sanitizing the Macedonian treasury

But what were the two reasons why Philip II had initially invaded Asia Minor (which was Persian territory)?

Well, to keep it short. Philip II was broke.

Philip II had not only extended the Macedonian borders and waged many wars that eventually put him into the position of Hegemon over the League of Corinth (a military alliance of almost all mainland and island greek city-states), but he had also reformed and professionalized the Macedonian army.

His problem was that all the wars with which Philip II extended the borders of Macedon as well as the professionalization and maintenance of the Macedonian army were extremely expensive and put a heavy burden on his treasury.

The empty treasury also explains why Philip II started his politics of expansion in the 340s BC. He needed to conquer and plunder new territories to unburden his strained state finances.

Just one example: When Alexander the Great became king of Macedon in late 336 BC, the treasury only had 50 talents of silver (1300 kg/ 2850 lbs) in it. But in contrast to that the treasury carried a debt of 500 talents (13000 Kg/ 28500 lbs). Or in other words. Alexander the Great was broke when he became king.

He needed money.

And Asia Minor (modern-day Turkey) was not only right across the Hellespont, but it was also pretty wealthy. So Philip II (and later Alexander the Great) hoped that they could get rid of their debts and fill up the treasury by conquering and plundering Asia Minor (which was part of the Persian Empire).

Just for comparison, each year the Persian king received a total tribute of 10,000 talents (260,000 kg/ 560,000 lbs) of silver. And an additional reserve of 250,000 talents (6,500,000 kg/ 14,000,000 lbs of silver) was stored in the Royal Persian residences for emergencies.

Needless to say that conquering just a relatively small (but rich) part of the Persian Empire like Asia Minor would have ended the Macedonian financial worries.

And the opportunity for a war with the Persian Empire was good. The Persian king Artaxerxes died in 338 BC (about 2 years before Philip started his attack on the Persian Empire) but because of political turbulences, the new Persian king, Darius III, only climbed the throne in 336 BC. And the new Persian king Dareios III only came from a sideline of the Royal Persian family.

So the Persian monarchy was weakened and the political turbulences in the Persian Empire promised a relatively easy victory. Additionally, expeditions like the March of the Ten Thousand had proven that even a relatively small but professional army could do well in Asia.

Do you want to find out more about the March of the Ten Thousand*, what they did in the Persian Empire, their Spartan commander, and why/ how Xenophon (a pupil of the famous Socrates) eventually became the leader of the Ten Thousand? Then I would like to recommend you the translated version of the Anabasis* in which Xenophon himself reports about the March of the Ten Thousand.

And that brings us to the last reason why Philip II started the war against the Persian Empire in 336 BC.

The Persians were simply an ideal enemy for his purposes.

The Persians as ideal enemies to solidify the Macedonian dominion over Greece

Philip II had been the most powerful man in Greece ever since late 346 BC. And especially the orator and political publisher Isokrates now demanded that all Greek city-states should rally under Macedonian leadership in a war against the Persian Empire.

The Persian Empire had been an old enemy ever since the Greco-Persian Wars. Do you want to find out more about how the Greco-Persian Wars started? Then I would like to recommend you my article here.

Because of the old hostilities, it was easy to call for a war against the Persian Empire to take revenge for the previous destructions in Greece by the Persians (like when the Persians burned down Athens in 480 BC). Another popular propaganda tool was to call for the liberation of the Greek city-states in Asia Minor who had been under Persian supremacy since 540 BC.

The political upside of a victory over the old Greek enemy Persia would have been huge since it would have reinforced the legitimation of Philips’s hegemony over the Greek city-states!

That kind of upside combined with the possibility of plundering the rich Asia Minor as well as the assumption that the Persian Empire was weakened by the new king Darius III, who only came from a sideline of the Achaemenid family convinced Philip II to attack Asia Minor in early 338 BC. That started the war between Macedonia and the Persian Empire which Alexander the Great then inherited in the fall of 336 BC.

And the rest is history. Within 12 years Alexander did not only conquer the Persian Empire, but he also marched all the way east into India (or at least the land the Greeks called India).

However, while Alexander conquered large parts of Asia he never turned west to conquer Rome and Italy. Here you can find out why Alexander the Great never conquered Rome and Italy.

But Alexanders’ empire would not outlast him. Almost immediately after Alexander the Great had died, his generals and friends started to fight over his empire. These fights, the Wars of the Dioadochi, would last for decades and reshape the entire Hellenistic world until 3 realms eventually rose from Alexanders’ former empire.

But that is a story for another time.

Take care of yourself because you deserve it. You really do.

Until next time

Yours truly

Luke Reitzer


Peter Green, Eugene N. Borza: Alexander of Macedon, 356–323 B.C.: A Historical Biography (2013).*

Xenophon: Anabasis.*

Arrian: Alexander the Great: The Anabasis and the Indica.*

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