Herodotus considered the whole of Babylonia to be a mere district of Assyria.

Pliny reckoned to it all Mesopotamia.

Strabo gave it, besides these regions, a great portion of Mount Zagros (the modern Kurdistan), and all Syria as far as Cilicia, Judaea, and Phoenicia.
If, leaving the conventional, which is thus vague and unsatisfactory, we seek to find certain natural limits which we may regard as the proper boundaries of the country, in two directions we seem to perceive an almost unmistakable line of demarcation.

On the east the high mountain-chain of Zagros.

penetrable only in one or two places, forms a barrier of the most marked character, and is beyond a doubt the natural limit for which we are looking.