The Battle of Gaugamela took place in 331 BC in what is now Iraqi Kurdistan, possibly near Erbil, and resulted in a decisive victory for the Macedonians. After the Siege of Gaza, Alexander advanced from Syria towards the heart of the Persian empire, crossing both the Euphrates and the Tigris rivers without any opposition. Darius was building up a massive army, drawing men from the far reaches of his empire, and planned to use sheer numbers to crush Alexander. Though Alexander had conquered part of the Persian empire, it was still vast in area and in manpower reserves, and Darius could recruit more men than Alexander could dream of. Also present in the Persian army, a sign that the Persians were still very powerful, were the feared war elephants. While Darius had a significant advantage in a number of soldiers, most of his troops weren't as organized as Alexander's. As Result, Alexander gained Babylon, half of Persia, and all other parts of Mesopotamia.
Alexander founded a new Greek city, Alexandria and Early in 331 BC he was ready to depart and led his forces away to Phoenicia. He left Cleomenes of Naucratis as the ruling nomarch to control Egypt in his absence. Alexander never returned to Egypt.