Passing the Zagros chain, probably by way of Behistun and Kermanshaw, he easily retook Ecbatana, which was an open town, and undefended by the Parthians, and proceeded to prepare for a further advance eastward.

The route from Ecbatana to the Caspian Gates crosses, of necessity, unless a considerable circuit be taken, some large tracts of barren ground, inlets or bays of the Great Salt Desert of Iran.
Artabanus cherished the hope that here the difficulties of the way would effectually bar his enemy's progress, more especially as his troops were so numerous, and as water was scanty throughout the whole region.

The streams which flow from Zagros towards the East are few and scanty; they mostly fail in summer, which, even in Asia, is the campaigning season; and those who cross the desert at this time must depend on the wells wherewith the more western part of the region is supplied by means of _kanats_ or underground conduits, which are sometimes carried many miles from the foot of the mountains.

The position of the wells, which were few in number, was known only to the natives; and Artabanus hoped that the Syrian monarch would be afraid to place the lives of his soldiers in such doubtful keeping.

When, however, he found that Antiochus was not to be deterred by any fears of this kind, but was bent on crossing the desert, he had recourse to the barbaric expedients of filling in, or poisoning, the wells along the line of route-which the Syrian prince was likely to follow.