From about the close of the third century B.C.

symptoms of such an accumulation had begun to display themselves in the tract immediately north of the Jaxartes, and the inhabitants of the countries south of that river had suffered from a succession of raids and inroads, which were not regarded as dangerous, but which gave constant annoyance.
Crossing the great desert of Kharesm by forced marches, some of the hordes invaded the green valleys of Hyrcania and Parthia, and carried desolation over those fair and flourishing districts.

About the same time other tribes entered the Bactrian territory and caused alarm to the Greek kingdom recently established in that province.

It appears that the Parthian monarchs, unable to save their country from incursions, consented to pay a sort of black-mail to their invaders, by allowing them the use of their pasture grounds at certain fixed times--probably during some months of each year.

The Bactrian princes had to pay a heavier penalty.