We never hear of their doing so.

Probably, like the Assyrians, they made their conquests with armies composed entirely of native soldiers, or of those combined with such forces as were sent to their aid by princes in alliance with them.
The weakness arising from this lack of organization was increased by a corruption of manners, which caused the Medes speedily to decline in energy and warlike spirit.

The conquest of a great and luxurious empire by a hardy and simple race is followed, almost of necessity, by a deterioration in the character of the conquerors, who lose the warlike virtues, and too often do not replace them by the less splendid virtues of peace.

This tendency, which is fixed in the nature of things, admits of being checked for a while, or rapidly developed, according to the policy and character of the monarchs who happen to occupy the throne.
If the original conqueror is succeeded, by two or three ambitious and energetic princes, who engage in important wars and labor to extend their dominions at the expense of their neighbors, it will be some time before the degeneracy becomes marked.

If, on the other hand, a prince of a quiet temper, self-indulgent, and studious of ease, come to the throne within a short time of the original conquests, the deterioration will be very rapid.