Hitti, Andreas Stratos, D. R. Hill and Khalil Athamina date his dispatch to after Abu Bakr's death. In the comprehensive 9th-century history of the early Muslim conquests by al-Baladhuri, the latter states "there is no truth" to the claim Abu Ubayda was sent by Abu Bakr; rather, the caliph "intended to send Abu ʿUbayda at the head of one of the armies, but the latter asked the caliph to relieve him of this mission". Athamina assesses that "certain allusions" in the Islamic sources offer context to the notion that Abu Ubayda, despite his participation in several expeditions under Muhammad and his high-standing among the Muslims, did not have the desire nor the necessary military experience and merit to accept the post Abu Bakr offered him. Modern research indicates that Abu Ubayda was dispatched to the Syrian front by Abu Bakr's successor, Umar, and early Muslim authors al-Baladhuri, al-Fasawi (d. 890) and Ibn Asakir (d. 1175) mention it was in the capacity of commander of an army of reinforcements. His arrival most likely dated to around 636, shortly after the first Muslim capture of Damascus in late 635 or during the preparation for the subsequent Battle of the Yarmuk. At the time, supreme command of the Muslim armies in Syria was held by Khalid ibn al-Walid. Umar may have sent Abu Ubayda to assume the supreme command. Several accounts in the Islamic tradition claim Abu Ubayda concealed the caliph's order from the rest of the army to avoid potentially insulting Khalid or provoking a mutiny while the Muslims were on the cusp of a major confrontation with the Byzantines. Athamina dismisses the reliability of these claims, considering them militarily illogical and meant to dramatize the change in command and emphasize Abu Ubayda's "moral superiority and unselfishness". Instead, Athamina maintains Abu Ubayda's appointment to the supreme command was made by Umar, who had kept in constant contact with Abu Ubayda through letters and emissaries, after the decisive Muslim victory at the Yarmuk. Abu Ubayda may have been chosen to lead at that time, when the Byzantine defense of Syria had taken an enormous blow, as the circumstances called for an able administrator to take the helm from a military commander like Khalid. The Islamic tradition provides a host of moral and personal reasons why Khalid was demoted in favor of Abu Ubayda, but most modern historians view these as either partially valid