The Abbasid Revolution in 750 CE challenged the political and social privileges held so far by the Arabs. The key figure in this revolution was Abu Muslim Khorasani. He was a Persian, born in Isfahan and therefore had impeccable credentials of birth with the exploited Persian majority. The legacy of Umayyad excesses had created extreme bitterness among the local population. Unfair taxation had fostered dislike of the Arabs among the Persians. Under the Abbasid rulers of the 9th century, the non-Arab converts comprised an important part of the army. The institution of wala' as a requirement to enter Muslim society ceased to exist after the fall of the Umayyads, as the Abbasids favoured a universal interpretation of Islam that was not the exclusive religion of the Arab elite. However, the rise to political power of non-Arab ethnic groups eventually restricted the power of the Abbasid caliph in Baghdad, as Persian, Turkic and Berber Muslims began to form independent sultanates.