This was throwing out a challenge which the Syrian monarch, Callinicus, could scarcely decline to meet, unless he was prepared to lose, one by one, all the outlying provinces of his empire.
Accordingly in B.C.237, having patched up a peace with his brother, Antiochus Hierax, the Syrian monarch made an expedition against Parthia.
Not feeling, however, altogether confident of success if he trusted wholly to his own unaided efforts, he prudently entered into an alliance with Diodotus the Bactrian king, and the two agreed to combine their forces against Tiridates.

Hereupon that monarch, impressed with a deep sense of the impending danger, quitted Parthia, and, proceeding northwards, took refuge with the Aspasiacae, a Scythian tribe which dwelt between the Oxus and the Jaxartes.

The Aspasiacae probably lent him troops; at any rate, he did not remain long in retirement, but, hearing that the Bactrian king, whom he especially feared, was dead, he contrived to detach his son and successor from the Syrian alliance, and to draw him over to his own side.

Having made this important stroke, he met Callinicus in battle, and completely defeated his army.
This victory was with reason regarded by the Parthians as a sort of second beginning of their independence.

Hitherto their kingdom had existed precariously, and as it were by sufferance.