…Alexander crosses northern Mesopotamia toward the Tigris, and Darius, learning of this move from an advance force sent under Mazaeus to the Euphrates crossing, marches up the Tigris to oppose him.
In an attempt to stop Alexander's incursion into the Persian empire, Darius prepares a battleground on the Plain of Gaugamela, between Nineveh and Arbela (present-day Irbil in northern Iraq), and posts his two hundred and fifty thousand troops to await Alexander's advance.
Darius has the terrain of the prospective battlefield smoothed level so that his many chariots can operate with maximum effectiveness against the Macedonians.
Alexander's well-trained but greatly outnumbered army faces Darius' massive battle line on October 1, 331 BCE, and organizes for attack, charging the left of the Persians' line with archers, javelin throwers, and cavalry, while defending against Darius' outflanking cavalry with reserve flank guards.
During the combat, so much of Darius' cavalry on his left flank are drawn into the battle that they leave exposed the Persian infantry in the center of the battle line.
Alexander and his personal cavalry immediately wheel half left and penetrate this gap, then wheel again to attack the Persians' flank and rear.
At this, Darius turns his chariot and flees, as at Issus.
Although his subordinates fight on, panic spreads through his entire army, which soon begins a headlong retreat while being cut down by the pursuing Greeks.
The Macedonians suffer fewer than five hundred casualties; the Persians lose an estimated forty thousand to ninety thousand men.
Alexander pursues the defeated Persian forces for thirty-five miles to …
