The Islamics know that the moon god of the Arabs isn't really the natural religion for the Persian people whose nature is more tolerant and peaceful and intelligent. Plus the Persians aren't really that fond of Arabs even though they could not defend their country against them and have lived under their influence and domination for centuries now. The religions of the Persian people have to be stomped out --- or they will replace Islam and the Arabs lose some of their power in the Middle East.
If Iran returns to Zoroastrianism,t hat would be the turning point for the war onIslam