Every great general has at least one decisive battle around which the course of history turns. For Alexander the Great, that battle took place near Arbela, or modern day Irbril in Iraq, on the plain of Gaugamela of Oct 1, 331 B.C.
Alexander, King of Macedon, had already defeated armies of the Persian Empire during a campaign that had led him and his army through modern day Turkey, down the coast of modern day Lebanon, Israel, and Syria, into Egypt, and then back into modern day Iraq. He had won the Battles of Granicus and Issus and had taken numerous towns and territory in the western part of the Persian Empire.
Gaugamela was a flat plain that was well suited for the superior Persian numbers, with its cavalry and scythed chariots. Alexander faced Persian host with 40,000 infantry and 7,000 cavalry. Arrian, the Roman historian who had access to primary sources, puts the Persian numbers at over 1 million, likely an exaggeration considering the problems of supplying such numbers in the ancient world.
Alexander opened with an attack on the Persian center with his Macedonian phalanx while he road with his cavalry and light infantry to attempt to open a gap in the Persian line by causing it to extend to stop him from out flanking him. The Persians launched at attack on the Macedonian line with its chariots which was easily defeated by javelin armed troops.
While the phalanx engaged the Persians in the center a gap, as Alexander expected, opened up in the line. He gathered his cavalry into a tight wedge and, with the light infantry following, rode through it, penetrating the Persian line and shattering it. Darius, the Persian king, fled with some few of his retainers. Alexander was obliged to let him go in order to ride to the air of the Macedonian infantry that was beset on the left flank.
Why Arbela was decisive
After each of Alexander’s previous victories, the Persians were able to regroup and gather more forces from their vast empire to continue resistance. This was not the case after Arbela. The credibility of King Darius was shattered and he was eventually murdered by his own retainers. Without a strong Persian successor, Alexander became King of Persian in his place. Even though Alexander’s empire did not outlive him, Arbela put an end to the Persian Empire for five centuries, after which it was reestablished under the Sassanid Dynasty in the year 224 AD.