Battle of Yarmouk

A Muslim army led by Khalid ibn al-Walid and Abu Ubayda ibn al-Jarrah fought the Byzantine army on the Yarmouk River in Syria. The battle lasted several days in August 636 CE. The Byzantines suffered heavy losses, and their defeat opened most of Syria to Muslim rule. Damascus surrendered soon afterward. Sources give very different figures for the size of the armies, and historians treat many of them with caution.