Several commentators, like John Calvin, see the phrase "a drought is upon her waters" as a remarkably specific prophecy. Historically, Cyrus the Great conquered Babylon by diverting the Euphrates River, effectively drying up the waterways that served as the city's primary defense. This allowed his army to walk into the city on the dry riverbed. The prophecy didn't just predict Babylon's fall, but the very method by which it would happen.