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. Under the new Arab elite of the Rashidun and later the Umayyad caliphates, both converted (mawali) and non-converted (dhimmi) Iranians were discriminated against, being excluded from the government and military, and having to pay a special tax called Jizya. Gunde Shapur, home of the Academy of Gunde Shapur which was the most important medical center of the world at the time, survived after the invasion, but became known as an Islamic institute thereafter.