Alexander began his journey at which city?

Thapsacus

Explanation:

 July 331 Alexander was at Thapsacus on the Euphrates. Instead of taking the direct route down the river to Babylon, he made across northern Mesopotamia toward the Tigris, and Darius, learning of this move from an advance force sent under Mazaeus to the Euphrates crossing, marched up the Tigris to oppose 

(brainliest please)

sumerians were responsible.