suggestion, Abu Ubayda ordered all the Muslim armies in Syria to evacuate the conquered land and concentrate at Jabiya. This maneuver gave a decisive blow to the Heraclius's plan, as he did not wish engage his troops in an open battle with the Muslims, where the light cavalry could be effectively used. From Jabiya, on Khalid's suggestion, Abu Ubayda ordered the Muslim army to withdraw on the plain of the Yarmouk River, where cavalry could be used. While the Muslim armies were gathering at Yarmouk, Khalid intercepted and routed the Byzantine advance guard. This was to ensure the safe retreat of the Muslims from conquered land. The Muslim armies reached there in July 636. A week or two later, around mid July, the Byzantine army arrived. The Byzantine commander in chief, Vahan, sent Christian Arab troops of the Ghassanid king, Jabalah ibn al-Aiham, to check the strength of the Muslims. Khalid's mobile guard defeated and routed the Christian Arabs; this was the last action before the battle started. For the next month negotiations continued between the two armies, and Khalid went to meet Vahan in person at Byzantine camp. Meanwhile, the Muslims received reinforcements sent by Caliph Umar. Finally on 15 August, the Battle of Yarmouk was fought, it lasted for 6 days and ended in a devastating defeat for the Byzantines. The Battle of Yarmouk is considered to be one of the most decisive battles of history. It was the historic defeat that sealed the fate of Byzantines, the magnitude of defeat was so intense that Byzantine could never recover from it. It left whole of the Byzantine Empire vulnerable to the Muslims. The battle was the greatest battle ever fought on Syrian soil till then and was a tactical marvel of Abu Ubayda. Siege of Jerusalem With the Byzantine army shattered and routed, the Muslims quickly recaptured the territory that they conquered prior to Yarmouk. Abu Ubayda held a meeting with his high command, including Khalid, to decide on future conquests. They decided to conquer Jerusalem. The Siege of Jerusalem lasted four months after which the city agreed to surrender, but only to caliph Umar in person. 'Amr ibn al-'As suggested that Khalid should be sent as caliph, because of his very strong resemblance to Umar. Khalid was recognized and eventually, Umar came and the Jerusalem surrendered on April 637. After Jerusalem, the Muslim armies broke up once