Province after province of their kingdom was swallowed up by the northern hordes, who gradually occupied Sogdiana, or the tract between the lower Jaxartes and the lower Oxus, whence they proceeded to make inroads into Bactria itself.

The rich land on the Polytimetus, or Ak Su, the river of Samarkand, and even the highlands between the upper Jaxartes and upper Oxus, were permanently occupied by the invaders; and if the Bactrians had not compensated themselves for their losses by acquisitions of territory in Afghanistan and India, they would soon have had no kingdom left.

The hordes were always increasing in strength through the influx of fresh immigrants, and in lieu of Bactria a power now stood arrayed on the north-eastern frontier of the Parthians, which was reasonably regarded with the most serious alarm and suspicion.
The origin of the state of things here described is to be sought, according to the best authorities, in certain movements which took place about B.C.200, in a remote region of inner Asia.

At that time a Turanian people called the Yue-chi were expelled from their territory on the west of Chen-si by the Hiong-nu, whom some identified with the Huns.
The Yue-chi separated into two bands; the smaller descended southwards into Thibet; the larger passed westwards, and after a hard struggle dispossessed a people called 'Su' of the plains west of the river of Hi.
These latter advanced to Ferghana and the Jaxartes; and the Yue-chi not long afterwards retreating from the Usiun, another nomadic race, passed the 'Su' on the north and occupied the tracts between the Oxus and the Caspian.

The Su were thus in the vicinity of the Bactrian Greeks; the Yue-chi in the neighborhood of the Parthians.