Alexander Defeats the Persians, 331 BC

Alexander began his war against the Persians in 334 BC. At the time the Macedonian leader was twenty-two years old. At his death eleven years later, Alexander ruled the largest empire of the ancient world. His victory at the battle of Gaugamela on the Persian plains was a decisive conquest that insured the defeat of his Persian rival King Darius III.

Link: http://www.eyewitnesstohistory.com/alexander.htm