With 2 large rivers at the same latitude as the Mississippi River basin, which is one of the most, if not most, agriculturally productive regions on Earth, one might wonder why Iraq is mostly a desert.
Salinization. Irrigation was key to expanding crop production to support the various city-states/kingdoms. The intense heat in Mesopotamia would quickly evaporate the water in the fields which would leave the mineral salts on the surface, slowly poisoning the soil. Over time this builds up so that the land can no longer be farmed.