The 1st October is the anniversary of the battle of Gaugamela, where Alexander the Great defeated Darius III. The exact site of the battle, as with so many ancient battles, is uncertain, but in took place not far from Mosul in what is now northern Iraq - the area can be seen on Google Earth here. Darius' army was annihilated, and he fled.
Although Darius tried to raise another army he was assassinated in 330 by Bessuss, the Satrap of Bactria and one of the commanders at Gaugamela. According to Curtius Rufus, Bessus was in turn crucified by Alexander at the same spot where he had murdered Darius.
The Achaemenid Empire fell as a result of this battle.