It is one of the great ironies of history that Emperor Heraclius, who rescued the Byzantine Empire from potential collapse at the hands of the Sassanid Empire, should preside over the defeat of the Byzantine army at the hands of the early Arab caliphs. The collapse of Byzantium’s military position in the near-east was sealed by the Battle of Yarmuk (also spelled Yarmouk) in AD 636.
Indeed, it is no exaggeration to state that the Battle of Yarmuk was one of the most decisive battles in history. In the course of six days, a vastly outnumbered Arab army succeeded in annihilating a significantly larger Byzantine force. This defeat led to the permanent loss of not only Syria and Palestine, but also of Egypt and large portions of Mesopotamia, and contributed in part to the rapid collapse of Byzantium’s traditional rival, the Sassanid Empire.