Arsaces III., who continued on the throne for about ten or twelve years after his peace with Antiochus, and Priapatius, or Arsaces IV., his son, who succeeded him, and had a reign of fifteen years, were content, as already observed, to watch over their own State, husbanding its resources, and living at peace with all their neighbors.

It was not till Phraates I.
(Arsaces V.), the son of Priapatius, had mounted the throne, B.C.

181, that this policy was departed from, and Parthia, which had remained tranquil for a quarter of a century, once more aroused herself, and assumed an attitude of aggression.
The quarter to which Phraates I.directed his arms was the country of the Mardians, a poor but warlike people, who appear to have occupied a portion of the Elburz range, probably that immediately south of Mazanderan and Asterabad.

The reduction of these fierce mountaineers is likely to have occupied him for some years, since their country was exceedingly strong and difficult.

Though the Mardi were (nominally, at any rate) subjects of the Seleucidae, we do not hear of any assistance being rendered them, or, indeed, of any remonstrance being made against the unprovoked aggression of the Parthian monarch.