Khalid subsequently moved against the largely Christian Arab tribes and the Sasanian Persian garrisons of the Euphrates valley in Iraq. He was reassigned by Abu Bakr to command the Muslim armies in Syria and he led his men there on an unconventional march across a long, waterless stretch of the Syrian Desert, boosting his reputation as a military strategist. As a result of decisive victories against the Byzantines at Ajnadayn (634), Fahl (634), Damascus (634–635) and Yarmouk (636), the Muslims under Khalid conquered much of Syria. He was afterward demoted from the high command by Umar for a range of causes cited by traditional Islamic and modern sources. Khalid continued service as the key lieutenant of his successor Abu Ubayda ibn al-Jarrah in the sieges of Homs and Aleppo and the Battle of Qinnasrin, all in 637–638, which collectively precipitated the retreat from Syria of imperial Byzantine troops under Emperor Heraclius. Umar dismissed Khalid from his governorship of Qinnasrin afterward and he died in Medina or Homs in 642.