The expeditions of Cyrus against the Massagetse, of Darius Hystaspis against the European Scyths, of Alexander against the Getee, of Trajan and Probus across the Danube, were designed to check and intimidate the northern nations, to break their power, and diminish the likelihood of their taking the offensive.

It was now more than four centuries since in this part of Asia any such effort had been made; and the northern barbarians might naturally have ceased to fear the arms and discipline of the South.

Moreover the circumstances of the time scarcely left them a choice.

Pressed on continually more and more by the newly-arrived Su and Yue-chi, the old inhabitants of the Transoxianian regions were under the necessity of seeking new settlements, and could only attempt to find them in the quarter towards which they were driven by the new-comers.

Strengthened, probably, by daring spirits from among their conquerors themselves they crossed the rivers and the deserts by which they had been hitherto confined, and advancing against the Parthians, Bactrians, and Arians, threatened to carry all before them.
We have seen how successful they were against the Bactrians.