Here Khalid took over the command of the Muslim armies in Syria from Abu Ubaidah, according to the instructions of the Caliph. Massive Byzantine armies were concentrating at Ajnadayn to push the invading armies back to the desert. Early Muslim sources claim the Byzantine strength to have been 90,000, although most modern historians doubt the figures, yet consider this battle to be the key to breaking Byzantine power in Syria. On Khalid's instructions, all Muslim corps concentrated at Ajnadayn, where they won a decisive battle against the Byzantines on 30 July.