There were two reasons of invasion on Kushana empire by the ruler of Parthia
1. Bactria was an important place from trading point of view, so he wanted to establish his control over it.
2. Ariana Pradesh was earlier under the control of Parthia but later on the Kushanas established their control over it, and the ruler of Parthia wanted to establish his control again over the Ariana Pradesh. As a result of this battle the whole Parthia Pradesh became the part of the Kushana kingdom.
