A ranar 22 ga Agusta, Abubakar, Halifan Rashidun na farko, ya rasu, bayan ya nada Umar a matsayin magajinsa.Yunkurin farko da Umar ya yi shi ne ya sauke Khalid daga kwamanda ya nada Abu Ubaidah bn al-Jarrah a matsayin sabon babban kwamandan sojojin Musulunci.Khalid ya yi alkawarin biyayya ga sabon Halifa kuma ya ci gaba da zama kwamanda na yau da kullun a karkashin Abu Ubaidah.An ruwaito yana cewa: “Idan Abubakar ya rasu, Umar kuwa halifa ne, to mu mun ji kuma mu yi biyayya”.Abu Ubaidah ya kara tafiya a hankali a hankali, wanda hakan ya yi tasiri a ayyukan soji a Siriya.Abu Ubaidah, kasancewarsa masoyin Khalid, ya nada shi kwamandan dawakai, ya kuma dogara da nasiharsa a duk lokacin yakin.