<P> The border forts of Sawa, Arak, Tadmur, Sukhnah, al - Qaryatayn and Hawarin were the first to fall to the invading Muslims . Khalid marched on to Bosra via the Damascus road . At Bosra, the Corps of Abu Ubaidah and Shurhabil joined Khalid, after which here as per orders of Caliph Abu Bakr, Khalid took the high command from Abu Ubaidah . Bosra was not ready for this surprise attack and siege, and thus surrendered after a brief siege in July 634 (see Battle of Bosra), this effectively ending the Dynasty of the Ghassanids . </P> <P> From Bosra Khalid sent orders to other corps commanders to join him at Ajnadayn, where according to early Muslim historians, a Byzantine army of 90,000 (modern sources state 9,000) was concentrated to push back the Muslims . The Byzantine army was defeated decisively on 30 July 634 in the Battle of Ajnadayn . It was the first major pitched battle between the Muslim army and the Christian Byzantine army and cleared the way for the Muslims to capture central Syria . Damascus, the Byzantine stronghold, was conquered shortly after on 19 September 634 . After the Muslim Conquest of Damascus, the Byzantine army was given a deadline of 3 days to flee as far as they could, with their families and treasure, or simply agree to stay in Damascus and pay tribute . </P> <P> After the three - day deadline was over, the Muslim cavalry under Khalid's command attacked the Roman army by catching up to them using an unknown shortcut at the battle of Maraj - al - Debaj . </P> <P> On 22 August 634 Abu Bakr died, making Umar his successor . As Umar became caliph, he relieved Khalid of command of the Islamic armies and appointed Abu Ubaidah ibn al - Jarrah as the new commander . The conquest of Syria slowed down under him while Abu Ubaida relied heavily on the advices of Khalid, and kept him beside him as much as possible . </P>

How far did east did muslim territory spread during the period of the rightly guided caliphs