After securing the eastern Mediterranean seaboard and Egypt, Alexander the Great pushed east into Mesopotamia with the intention of bringing Darius to battle. After crossing the Euphrates River unopposed, he marched his army eastward along the foothills of the Armenian mountains before crossing the Tigris River. Once across the Tigris, Macedonian mounted scouts reported seeing Persian cavalry in the distance. Taking no chances, Alexander drew his army up in battle order, and, whilst the bulk of his troops cautiously advanced, he personally led a contingent of horsemen and light infantry against these Persians, scattering them and taking prisoners. He deduced from his captives that Darius was awaiting him at Gaugamela, a small village beside the river Bumodus.
This is a companion discussion topic for the original content at https://www.worldhistory.org/article/108/the-battle-of-gaugamela-331-bce/.