After Khalid consolidated his control of Iraq, Abu Bakr sent four armies to Syria on the Byzantine front under four different commanders: Abu Ubaidah ibn al-Jarrah (acting as their supreme commander), Amr ibn al-As, Yazid ibn Abu Sufyan and Shurhabil ibn Hasana. However, their advance was halted by a concentration of the Byzantine army at Ajnadayn. Abu Ubaidah then sent for reinforcements. Abu Bakr ordered Khalid, who by now was planning to attack Ctesiphon, to march from Iraq to Syria with half his army. There were 2 major routes to Syria from Iraq, one passing through Mesopotamia and the other through Daumat ul-Jandal. Khalid took an unconventional route through the Syrian Desert, and after a perilous march of 5 days, appeared in north-western Syria.