The conquest of the Persian Empire was something that could have easily failed. Even though they relied on tribal levies, were constantly fighting rebellions, and dealt with infighting for the throne, they still were the greatest empire in the world at the time. A Greek city state standing alone stood no chance against it and even the classical hoplites that beat them at Marathon and Plataea would not make it far into the Persian heartland. The Macedonian military was a professional well trained organization and they were able to maneuver in ways that could offset the numbers the Persian king could bring to the field. The traditionally strong and well trained Hetaroi were able to smash through Persian cavalry and the Pezetairoi have had thirty years training in warfare and were able to outfight any conscripted man who was more of a peasant than a soldier. Alexander was also just as great of a tactician as his father. He was able to adapt to situations and make calls that even went against the advice of his officers. He was the only man who could bring his troops 20,000 miles in 12 years.

The Persian Empire fell due to a combination of these reasons. A weakened Persia was not able to be taken advantage of by a separated Greece. The Persians went up against Macedonia, which had not only a well experienced, competent military, but a young charismatic, military genius. Maybe Asia was destined to fall on the day the Temple of Diana burned to the ground, but it was only possible under all of the circumstances lining up perfectly.