A lunar eclipse occured on the evening of September 20, 331 B.C., eleven days before the battle of Gaugamela in which Alexander the Great defeated Darius III, king of Persia. The eclipse came to be seen as an omen of this victory.

From Curtius Rufus Quintus,  De Rebus Gestis Alexandri Magni Cum Supplementis Freinshemii. (The History of Alexander). Book IV, Chapter 10.

'Alexander encamped there for two days and had marching orders proclaimed for the third, but at about the first watch there was an eclipse of the moon. First the moon lost its usual brightness, and then became suffused with a blood-red colour which caused a general dimness in the light it shed. Right in the brink of a decisive battle the men were already in a state of anxiety, and this now struck them with a deep religious awe which precipitated a kind of a panic.

They complained that the gods opposed their being taken to the ends of the earth, that now rivers forbade them access, heavenly bodies did not mantain their erstwhile brightness, and they were met everywhere by desolation and desert. The blood of thousands was paying for the grandiose plans of one man who despised his country, disowned his father Philip, and had deluded ideas about aspiring to heaven.

Mutiny was but a step away when, unperturbed by all this, Alexander summoned a full meeting of his generals and officers in his tent and ordered the Egyptian seers (whom he believered to possess expert knowledge of the sky and the stars) to give their opinion. They were well aware that the annual cycle follows a pattern of changes, that the moon is eclipsed when it passes behind the earth or is blocked by the sun, but they did not give this explanation, which they themselves knew, to the common soldiers. Instead, they declared that the sun represented the Greeks and the moon the Persians, and that an eclipse of the moon predicted disaster and slaughter for those nations. They then listed examples from history of Persian kings whom a lunar eclipse had demonstrated to have fought without divine approval.

Nothing exercises greater control over the masses than superstition. Usually ungovernable, cruel and capricious, when they are gripped by superstition they obey prophets more readily than their generals. Thus the dissemination of the Egyptians' responses restored hope and confidence to the dispirited soldiers. The king felt he should exploit this surge of confidence, and at the second watch he moved camp, keeping the Tigris on his right and the so-called Gordyaean Mountains on his left.'

Translation by J. Yardley. © Penguin Books.