After Muhammad died in 632, his successors (the Caliphs) continued to lead the Muslim community based on his teachings and guidelines of the Quran. The majority of Muslims consider the first four successors to be 'rightly guided' or Rashidun. The conquests of the Rashidun Caliphate helped to spread Islam beyond the Arabian Peninsula, stretching from northwest India, across Central Asia, the Near East, North Africa, southern Italy, and the Iberian Peninsula, to the Pyrenees. The Arab Muslims were unable to conquer the entire Christian Byzantine Empire in Asia Minor during the Arab–Byzantine wars, however. The succeeding Umayyad Caliphate attempted two failed sieges of Constantinople in 674–678 and 717–718. Meanwhile, the Muslim community tore itself apart into the rivalling Sunni and Shia sects since the killing of caliph Uthman in 656, resulting in a succession crisis that has never been resolved. The following First, Second and Third Fitnas and finally the Abbasid Revolution (746–750) also definitively destroyed the political unity of the Muslims, who have been inhabiting multiple states ever since. Ghaznavids' rule was succeeded by the Ghurid Empire of Muhammad of Ghor and Ghiyath al-Din Muhammad, whose reigns under the leadership of Muhammad bin Bakhtiyar Khalji extended until the Bengal, where Indian Islamic missionaries achieved their greatest success in terms of dawah and number of converts to Islam. Qutb-ud-din Aybak conquered Delhi in 1206 and began the reign of the Delhi Sultanate, a successive series of dynasties that synthesized Indian civilization with the wider commercial and cultural networks of Africa and Eurasia, greatly increased demographic and economic growth in India and deterred Mongol incursion into the prosperous Indo-Gangetic Plain and enthroned one of the few female Muslim rulers, Razia Sultana.