I really need some help on my Baktrian campaign, so if anyone has any suggestions tell me!

When i tried to take a rebel city with my only army, my ally Saka approached one of my towns, so i raced back to that town, and then the Sakas left! Then the Selucid sent a small army near Baktra, so my army has to go there. And then the army left!

Who should i attack? the Saka, selucid empire or parthia. the saka and the empire are allied to me and each other. the best choice would be to attack Parthia, but if i sent my only army there i wont have anyone to defend my own land.