The Battle of Yarmuk (also spelled Yarmuq or Hieromyax) took place between the Arabs and the Byzantine Empire in 636.
Byzantine emperor Heraclius organized a force of about 40 000 troops after the Arabs conquered Damascus and Emesa, and succeeded in recapturing those cities. Part of the Byzantine force under Theodore the Sacellarius was then defeated outside Emesa, and the Arabs under Khalid ibn Walid met the other Byzantine commander, Baänes in the valley of the Yarmuk River outside Damascus on August 20. Khalid was at first pushed back, but though his army was only about half the size of the Byzantine army, he eventually broke through, killing most of Baänes men and chasing the rest over a cliff. As a result, all of Syria was open to the Arabs, and Heraclius began to concentrate his army on a defense of Egypt instead.