Complex sentence: On 22 August 634, Caliph Abu Bakr died and Umar became caliph he relieved Khalid ibn al-Walid from the command of the Islamic army and appointed Abu Ubaidah as the new commander.
Split into simpler sentences: On 22 August 634, Caliph Abu Bakr died making Umar his successor. As Umar became caliph he relieved Khalid ibn al-Walid from the command of the Islamic army and appointed Abu Ubaidah as the new commander.