In 334 BC, Alexander, 21 year-old ruler of the small Greek kingdom of Macedonia, led

an invasion of the vast Persian Empire.

It seemed impossible odds, but thanks to Greek military dominance, and Alexander's fearless

leadership, he won two great battles against the Persians…

at the River Granicus, and at Issus.

Having subdued Persian lands west of the Euphrates River, he now headed east into the empire's

heartlands, seeking a final showdown with the Persian King, Darius III.

Receiving news that a great Persian army, led by Darius, had assembled at Gaugemela,

near modern Mosul in Iraq – he made straight for it.

This was Darius's last chance to stop Alexander – and Alexander's chance to smash Persian

power once and for all.

Darius had chosen to fight on open ground, where his advantage in numbers would be more

telling.

His soldiers had also worked hard to clear and flatten the terrain, to make it suitable

for Persian war chariots.

By modern estimates, the Persian Army was between 50 and 80,000 strong, and made up

of contingents from across the empire:

infantry from Syria and Babylonia...

cavalry from Armenia, India and Central Asia...

up to 200 scythed chariots... even a handful of war elephants.

Alexander's army was smaller, and may have been outnumbered by as much as two to one.

He deployed his units in their usual formation:

On the left flank, Thracian and Thessalian cavalry, commanded by Parmenion.

In the centre, the Macedonian veterans of the phalanx – each armed with their 18 foot

sarissa pike.

On the right flank, Alexander with his elite cavalry, the Companions; and his best infantry,

the hypaspists.

These were the units with which Alexander planned to launch his main attack.

Greek hoplites formed a second line, and supported both wings – which were angled back, to

guard against encirclement by the Persians.

The battle began when Alexander led his wing out to the right – a move that took the

Persians by surprise.

Could Alexander really be trying to encircle their huge army?

The Persians mirrored his movement, taking troops away from their centre, to outflank

Alexander, and prevent him leaving the area they'd cleared for the Persian chariots.

But Alexander's unusual manoeuvre was a trap - to entice the Persians to weaken their centre.

When he saw that it had worked, he ordered his Greek cavalry to charge, to keep the Persians

fixed in position.

A giant cavalry battle developed on the right wing.

Darius, meanwhile, judging this to be the decisive moment, unleashed his chariots.

But expert Agrianes javelin-throwers took out horses and crews – while the Greek infantry

opened lanes, allowing the chariots to pass harmlessly through.

Now Alexander led his Companion cavalry, and parts of the Macedonian phalanx, in a headlong

charge straight at the weakened Persian centre, fighting his way towards Darius himself.

The sudden ferocity of Alexander's assault threw the Persians into panic – the centre

of the army broke and ran - King Darius himself leading the rout.

But Alexander's left wing was in serious trouble – Parmenion, facing a huge onslaught by

Persian cavalry, was virtually surrounded - Indian and Scythian horsemen had even ridden

through a gap in the Greek line – but rather than wheeling and attacking the Greeks from

behind, they'd carried straight on to loot their camp.

Parmenion sent a desperate appeal to Alexander for help.

The King abandoned his pursuit of Darius, regrouped, and charged the Persian right wing.

It was the hardest and bloodiest fighting of the battle – claiming the lives of sixty

of Alexander's Companions.

Finally, as news of Darius's flight spread across the battlefield, the last Persian horsemen

turned and fled.

The Battle of Gaugamela was a stunning and complete victory for Alexander.

According to ancient sources, he lost just a few hundred men, while the Persians lost

thousands.

Alexander had routed Darius's great army, and now the road to Babylon – the empire's

main capital - lay open.

The Macedonian king entered the great city in triumph, recognised by Persian officials

as its new rightful ruler.

So too at the city of Susa, where Alexander ceremonially took his seat upon the royal

throne of Persia.

In the Zagros mountains, at a pass known as the Persian Gates, a courageous Persian force

held up Alexander's army for a month.

The Greeks eventually found a mountain path that bypassed their position, allowing them

to encircle and wipe out the defenders.

In early 330 BC, Alexander reached Persepolis, the empire's ceremonial capital.

Alexander wanted to appear as a liberator to the Persians – as a legitimate successor

to King Darius - but now, he ordered Persepolis to be pillaged and burnt – retribution for

the Persian invasion of Greece, and the burning of Athens' sacred temples in 480 BC.

Alexander now headed north into Media, where Darius had taken refuge in the royal city

of Ecbatana.

Alexander was determined to capture Darius – but the fugitive king fled east in the

hope of raising a new army in the provinces of Parthia, Bactria, and Sogdia.

It was not to be.

As Alexander closed in, the Persian king was murdered by one of his own governors, Bessus,

who then proclaimed himself the empire’s new ruler.

Alexander gave orders for Darius to be buried in the royal tombs of Persepolis, alongside

his ancestors.

Then he paused to organise his vast new empire.

Alexander appointed viceroys to rule the provinces on his behalf, keeping several Persians - who

had sworn loyalty - in their posts.

He also allowed Greek troops, who wished, to return home.

Then he resumed his march east.

His goal: to find and kill the usurper Bessus...

subjugate the empire's eastern provinces...

and reach the far edge of the world...

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