From Wikipedia

The Battle Of Gaugamela

The Battle of Gaugamela took place in 331 BC between Alexander the Great and Darius III of Persia. The battle, which is also called the Battle of Arbela, resulted in a massive victory for the Macedon and led to the fall of the Achaemenid Empire.

During the two years after the Battle of Issus, Alexander proceeded to occupy the Mediterranean coast and Egypt. He then advanced from Syria against the heart of the Persian empire. Alexander crossed 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. He planned to use numbers to crush Alexander, and according to some contemporary historians he gathered around 100,000 men. Darius also picked a flat plain for a battlefield so Alexander would have no advantages in terrain, and allowing Darius to use his vast horde more effectively.