Query: Which Arab Caliphate defeated Iran first?
Context: The prolonged Byzantine-Sassanid Wars, most importantly the climactic Byzantine-Sassanid War of 602-628, as well as the social conflict within the Sassanid Empire, opened the way for an Arab invasion to Iran in the 7th century. Initially defeated by the Arab Rashidun Caliphate, Iran came under the rule of the Arab caliphates of Umayyad and Abbasid. The prolonged and gradual process of the Islamization of Iran began following the conquest.