Today in history, on Aug. 20, 636, arguably the single most consequential battle between Islam and the West took place — that of Yarmuk. Occurring just four years after the Muslim prophet Muhammad had died, not only did the military engagement decide whether the Arabian creed thrives or dies; it became a chief source of inspiration and instruction for jihadis throughout the centuries, right down to the Islamic State. And yet very few in the West are even aware of the Battle of Yarmuk’s existence — much less how it motivates contemporary Islamic terrorists.