The First Caliph was Abu Bakr. Under his rule he established Islamic control over Arabia and made his campaign towards Syria. In 634, Bakr's successor, Umar, conquered Syria, part of the Byzantine Empire, Jerusalem, Damascus, Persia, and finally the Sasanian capital, Ctesiphon. During his rule he created different towns that separated the soldiers from townsmen. He also instituted the the Muslim calendar which started with the testimonies of Muhammad in 639. After the assassination of Umar, Uthman becomes the third caliph, but with much backlash from the Muslim community. There was a split between the Shia and Sunni because the Shia wanted the next caliph to be inherited, while the Sunni believed they should be selected. After Uthman died, Ali Ibn Sbi Talib became the fourth caliph.