As the main Byzantine defence line started from the coastal regions near Ghazahh, Yazid arrived at the Valley of Araba at about the same time as Amr bin Al Aas reached Elat. The two forward detachments sent by the Byzantine army to prevent the entry of Yazid's and Amr's corps, respectively, into Palestine, were easily defeated by them, though they did prevent the Rashidun forces from reaching their assigned objective. Abu Ubaidah and Shurhabil, on the other hand, continued their march, and by early May 634 they reached the region between Bosra and Jabiya. The Emperor Heraclius, having received intelligence of the movements of the Muslim armies from his Arab clients, began to plan countermeasures. Upon Heraclius' orders, Byzantine forces from different garrisons in the north started moving to gather at Ayjnadyn. From here they could engage Amr's corps and maneuver against the flank or rear of the rest of the Muslim corps that were in Jordan and Southern Syria. The strength of the Byzantine forces, according to rough estimates, was about 100,000. Abu Ubaidah informed the Caliph about the preparations made by the Byzantines in the third week of May 634. Because Abu Ubaida didn't have experience as a commander of military forces in such major operations, especially against the powerful Roman Army, Abu Bakr decided to send Khalid ibn Walid to assume command. According to early Muslim chronicles, Abu Bakr said, "By Allah, I shall destroy the Romans and the friends of Satan with Khalid Ibn Al Walid."