Abu Ubaidah ibn al-Jarrah
On 22 August, Abu Bakr, the first caliph, died, having made Umar his successor. Umar's first move was to relieve Khalid from command and appoint Abu Ubaidah ibn al-Jarrah as the new commander-in-chief of the Islamic army. Abu Ubaidah got the letter memorializing this during the siege, but he delayed the announcement until the city had been conquered. Later on, Khalid pledged his loyalty to the new Caliph and continued to serve as an ordinary commander under Abu Ubaidah. He is reported to have said, "If Abu Bakr is dead and Umar is Caliph, then we listen and obey."