In the Battle of Gaugamela in 331 BC Alexander the Great of Macedonia defeated Darius III of Persia. The battle is also inaccurately called the Battle of Arbela.
Macedonians and Greeks under Alexander, 7,000 cavalry and 40,000 infantry. (According to Arrian)
Persians under Darius, with maybe 40,000 cavalry, 200,000 infantry and 6,000 Greek mercenaries. (Arrian gives an exaggerated number of 1,000,000 infantry.) The actual Persian numbers are unknown, although it is fairly clear that they substantially outnumbered the Hellenic forces.
Darius chose (or smoothed out, depending on accounts) a flat plain where he could deploy his numerically superior cavalry forces. The location of the battle - near Tel Gomel, east of Mosul in northern modern-day Iraq - was determined by Sir Aurel Stein in 1938 (see his Limes Report, pp. 127-1. After the battle, Darius fled to Arbela (modern-day Arbil) 120 km to the east. The distance from Arbil to Mosul is roughly 80 km, so considerable room for discussion remains.
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.
The battle began with the Persians already present at the battlefield.
Darius had recruited the finest cavalry from his satrapies and from an allied Scythian tribe. Darius also deployed scythed chariots for which he had prepared cleared terrain in front of his troops. He also had 50 Indian elephants supported by Indian chariots.
Before the battle, Darius ordered bushes and vegetation removed from the battlefield, to maximize...